Classification of Non-Chordates

Phylum Porifera : The sponges (pore bearing animals)

(Gk. Porus = Pore ferre = To bear)

Brief History : Robert Grant (1825) finally proved that sponges are animals, and coined the name ‘Porifera’ for these. Schulze (1878), Butschli (1884), Sollas (1884) and Delage (1898) separated sponges from other metazoans on the basis of embryological studies, and suggested a separate group, “Parazoa” for these.

General Characters  

(1) All the sponges are aquatic, sedentary, asymmetrical or radially symmetrical. First multicellular organisms and have cellular grade of organization.

(2) They are diploblastic. Ectoderm is formed by pinachocyte and endoderm is formed by choanocyte. Both layers are called pinachoderm and choanoderm. A gelatinous noncellular mesenchyme present in between them.

(3) Mesenchyme contains free amoebocytes and skeletal elements.

(4) Different types of amoebocytes are :

Archaeocytes      :  undifferentiated totipotent cells.

Chromocytes       :  with pigment granules.

Thesocytes          : with reserve food granules.

Myocytes             :  highly contractile, spindle-shaped cells.

Trophocytes        :  supply nutrients to developing cells (nurse ꃎlls)

Gland cells           :  secrete slimy substance.

Sex cells             :ꃞvelop from archaeocytes only during breeding season.

(5) The body is perforated by numerous minute pores called ostia.

(6) The ostia open into a large cavity called spongocoel or paragastric cavity.

(7) The spongocoel opens to the outside by a large opening called osculum.

(8) Sponges have a canal system and they need a continuous current of water flowing through their bodies for respiration, excretion, nutrition and reproduction.

(9) Different types of canal system in sponges are asconoid, syconoid and leuconoid.

(10) The simplest type of canal system in porifera is asconoid type.

(11) The course taken by the water current way be shown as under –

Ingressing water Ostia→ Spongocoel Osculum→ To outside

(12) The sponges possess an endoskeleton in the form of calcareous spicules.

(13) Excretion and respiration occur by diffusion.

(14) They have greater power of regeneration due to totipotent archaeocytes.

(15) Digestion in sponges is intracellular like protozoans.

(16) All sponges are hermaphrodite, reproduction takes place by asexual or sexual methods.

(17) Gemmules are internal buds containing archaeocytes, mostly found in fresh water sponges, concerned with asexual reproduction.

(18) Development is indirect or direct. The common larval forms are parenchymula (leucosolenia and Clathrina), amphiblastula (Sycon), etc.

Classification of porifera : On the basis of types of endoskeleton, phylum porifera is divisible into three classes

Class 1. Calcarea or Calcispongiae

(1) Skeleton is formed of Calcareous spicules. 

(2) Radially symmetrical.

(3) Choanocyte cells are large and conspicuous.

(4) Canal system asconoid (ascon) or syconoid (sycon) type.

(5) These are also known as limy sponges.

Examples : Clathrina, Leucosolenia, Sycon, Grantia, etc.,

∙ Leucosolenia is a smallest sponge with asconoid type of canal system.

Class 2. Hexactinellida Or Hyalospongiae

(1) Skeleton is formed of six rayed triaxon, silicious spicules,

(2) Canal system is branched or unbranched.

(3) Radially symmetrical.

(4) These are also known as glass sponges.

Examples : Pheronema, Hyalonema, Euplectella, etc.,

∙ Euplectella is the sponge which is given as a Gift in Japan and known as “venus flower basket”. It show commensalism with shrimps of the genus spongicola, ‘life upto death’.

Class 3. Demospongia

(1) Skeleton either absent or present. When present it is either formed of spongin fibres or combination of spongin fibres and silicious spicules.

(2) The silicious spicules when present are never six rayed.

(3) The canal system is complicated Rhagon type or leuconoid type.

(4) Rhagon larva is formed.

(5) These sponges are of great economic importance.

Examples : Cliona, Spongilla, Chalina, Euspongia, Hippospongia, Oscarella, etc.,

∙ Spongilla is a fresh water sponge.

∙ Cliona is harmful to oyster industry.

Phylum-Cnidaria (Coelenterata)

(Gk. knide = nettle or stinging cell)

Brief History : Peyssonel (1723) and Trembley (1744) proved these to be animals. Hence, Linnaeus (1758), Cuvier (1796) and Lamarck (1801) included these under ‘Zoophyta’, together with sponges. Leuckart (1847) included sponges and cnidarians under his phylum Coelenterata. Finally, Hatschek (1888) divided 𠇌oelenterata” into three phyla–Spongiaria (= Porifera), Cnidaria and Ctenophora.

General characters 

(1) Coelenterates are radially symmetrical animals with tissue grade of body organization.

(2) All the members of this phylum are aquatic, mostly marine.

(3) They are solitary or colonial, sedentary or free swimming.

(4) The body wall is diploblastic. It is made up of two layers of cells, namely the ectoderm and the endoderm with a non-cellular layer called mesogloea in between.

(5) Cnidarians exhibit diamorphism with polypoid and medusoid stage (Metagenesis or alternation of generation).

(6) Asexual phase is generally polyp and sexual phase is medusa.

(7) Coelom is absent Hence coelenterates are acoelomate animals.

(8) A gastrovascular cavity or coelenteron is present. It can be compared to the gut of higher animals.

(9) Mouth is present but anus is absent (blind-sac body plan). Mouth is surrounded by tentacles.

(10) The most characteristic feature of coelenterates is the presence of nematocysts or stinging cells.

(11) Digestion is extracellular as well as intracellular.

(12) Respiratory, excretory and circulatory system are absent.

(13) Primitive nervous system with synaptic or non-synaptic nerve net but no brain.

(14) Sense organs are statocysts (tentaculocysts), ocelli and olfactory pits.

(15) Reproduction both asexual and sexual.

(16) Development is indirect as there are one or two larval forms, Planula (Obelia) and Ephyra (Aurelia).

Classification of coelenterata : On the basis of the dominance of medusoid or polypoid phase in the life cycle, phylum coelenterata is divided into three classes –

Class 1. Hydrozoa (Gr. hydros, water, zoios, animal)

(1) Hydrozoa are solitary and fresh water or mostly colonial and marine, sessile and free-swimming forms.

(2) They exhibit tetramerous or polymerous radial symmetry.

(3) Body wall consists of an outer ectoderm and an inner endoderm separated by a non-cellular gelatinous mesogloea.

(4) Gastrovascular cavity without stomodaeum, septa or nematocysts bearing gastric filament.

(5) Skeleton or horny structure is horny perisarc in some forms, while coenosarc secretes a skeleton of calcium carbonate forming massive stony structure or coral in other forms.

(6) They exhibit polymorphism. There are two main types of zooids, the polyp and medusa. Medusa is provided with true muscular velum.

(7) Many hydrozoa exhibit alternation of generation.

(8) Reproductive products of sex cells are usually ectodermal in origin and discharged externally.

(9) Cleavage is holoblastic, embryo ciliated planula.

(10) Both polypoid and medusoid stages present.

Examples : Hydra, Tubularia, Bougainvillea, Hydractinia, Eudendrium, Pennaria, Obelia, Sertularia, Plumularia, Companularia, Millepora, Stylaster, Geryonia, Physalia, Porpita, Velella, Pericolpa, Periphylla, Cynaea, Rhizostoma or Pilema Cassiopeia, etc.,

∙ Obelia is trimorphic and marine colony.

∙ Hydranth of obelia bears twenty four (24) tentacles while medusa bears sixteen (16) tentacles in addition to tentaculocysts.

Class 2. Scyphozoa (Gr. skyphos, cup, zoios, animal)

(1) Scyphozoa include large jellyfishes or true medusae.

(2) They are exclusively marine.

(3) Medusae are large, bell or umbrella-shaped and without true velum. They are free swimming or attached by an aboral stalk.

(4) Marginal sense organs are tentaculocysts.

(5) Polypoid generation is absent or represented by small polyp, the scyphistoma which gives rise to medusae by strobilization or transverse fission.

(6) Gastrovascular system is without stomodaeum, with gastric filaments and it may or may not be divided into four inter-radial pockets by septa.

(7) Mesogloea is usually cellular.

(8) Gonads are endodermal and the sex cells are discharged into the stomach.

Examples : Lucernaria, Haliclysus, Aurelia, Rhizostoma, Charybdea, Periphylla, Chrysaora.

∙ Rhizostoma is a polystomous scyphozoan with many mouth bearing structures called scapuletes.

Class 3. Anthozoa (Actinozoa)

(1) These are solitary or colonial exclusively marine forms.

(2) They are exclusively polypoid. Medusoid stage is altogether absent.

(3) Body is cylindrical with hexamerous, octomerous or polymerous biradial or radiobilateral symmetry.

(4) The oral end of the body is expanded radially into an oral disc bearing hollow tentacles surrounding the mouth in the centre.

(5) The stomodaeum is often provided with one or more ciliated grooves, the siphonoglyphs.

(6) Gastrovascular cavity is divided into compartments by complete or incomplete septa or mesenteries.

(7) Mesenteries bear nematocysts at their free edges.

(8) Mesogloea contains fibrous connective tissue and amoeboid cells.

(9) They are exclusively marine, many forms corals.

Subclass 1. Alcyonaria (Octocorallia)

(1) These are colonial marine forms.

(2) Polyps are long or short cylinders terminating orally into a flat circular oral disc having the oval or elongated mouth in the centre.

(3) Polyps always bear eight pinnate, hollow tentacles.

(4) Eight complete mesenteries are present.

(5) Single ventral siphonoglyph is present

(6) Endoskeleton is the product of mesogloeal cells comprised of calcareous spicules either calcareous or horny in nature.

(7) Polyps are dimorphic in some forms.

Examples : Tubipora, Clavularia, Alcyonium, Xenia, Heliopora, Gorgonia, Corallium,Testudo, etc.,

∙ Corals form rocks in the sea, called the coral reefs. The largest coral reef is the great barrier reef which is 1200 miles long and surrounds Australia complete.

Subclass 2. Zoantharia (Hexacorallia)

(1) These are solitary or colonial marine forms.

(2) Tentacles simple, rarely branched, hollow cone shaped, numerous arranged in the multiple of five and six but never eight.

(3) Mesenteries are numerous arranged in the multiple of five or six, may be complete or incomplete.

(4) Two siphonoglyphs are commonly present.

(5) Endoskeleton when present is calcareous, derived from ectoderm.

(6) Polyps are usually monomorphic.

Examples : Actinia, Metridium, Adamsia, Edwardsia, Astraea, Fungia, Zoanthus, Antipathes, Aeropora or Madrepora, etc.

∙ Metridium shows commensalism with Eupagurus.

Phylum-Ctenophora or Acnidaria- The comb Jellies

(Gk. kteis = comb pherein = To bear)

Brief History : The ctenophores as a distinct group were first recognized by Escscholtz (1829). Hatschek (1889) placed it under a separate phylum called ctenophora.

General characters

(1) All the ctenophores are marine.

(2) They are solitary and pelagic.

(3) They are transparent.

(4) The have tissue-grade of organization.

(5) They have biradial symmetry.

(6) They are acoelomate animals.

(7) They are unsegmented.

(8) They body-wall is diploblastic.

(9) The mesogloea contains cells.

(10) Nematocysts are absent.

(11) Special adhesive cells called colloblasts are present in all ctenophores.

(12) The gastrovascular system is well developed.

(13) Two anal openings are present.

(14) Skeletal system is absent.

(15) Excretion and respiration are carried out by diffusion.

(16) The nervous system is in the form of nerve net.

(17) An aboral sense organ is present in the form of statocyst.

(18) Cilia are used for locomotion.

(19) They are hermaphrodites.

(20) Development is indirect. It includes a cydippid larva.

Classification of Ctenophora

Class 1. Tentaculata

(1) The body is simple, rounded or oval or ribbon-like.

(2) Two long aboral tentacles are present.

(3) Mouth is narrow and pharynx is small.

Examples : Pleurobrachia, Hormiphora, Mertensia Mnemiopsis, Bolinopsis, Velamen, Cestum, Ctenoplana, Coeloplana, etc.

∙ Cestum is commonly called “venus’s girdle”.

∙ Ctenoplana shows commensal with Alcyonea. 

Class 2. Nuda

(1) Body is large thimble-shaped or conical.

(2) Tentacles are absent.

(3) Mouth is wide and pharynx is large.

(4) The meridional vessels are produced into a complex system of anastomosing branches.

Example : Beroe

∙ Befroe is commonly called “Swimming eye of cat”.

Phylum Platyhelminthes : The flat worms

(Gk. platys = broad or flat helmin = worm)

Brief History : Aristotle mentioned tapeworms, but scientific studies of flatworms began only in the 18th century. It was Gegenbaur (1859) who placed these in a separate group and suggested the present name of the phylum.

General Characters

(1) They are dorso-ventrally flattened like a leaf.

(2) They show organ grade of organization.

(3) They are acoelomate animals. The cavity in platyhelminthes is filled with mesenchyme or parenchyma.

(4) They are triploblastic animals. The cells of the body wall are arranged in three layers.They are the ectoderm, the mesoderm and the endoderm.

(5) They are bilaterally symmetrical animals. The body of the animal can be divided into two equal similar halves through only one plan. Animals with this symmetry have definite polarity of anterior and posterior ends.

(6) Some members have segmented body. The segmentation in platyhelminthes is called pseudometamerism.

(7) Many of the parenchyma cells give rise to muscle fibres.The muscle fibres are arranged in circular, longitudinal and vertical layers.

(8) The digestive system is completely absent from Cestoda and Acoela. The alimentary canal is branched in Turbellarians. The anus is absent from them.

(9) The respiratory organs are absent. In parasites respiration is anaerobic.

(10) There is no circulatory system.

(11) The excretory system is formed of protonephridia (flame cells or solenocytes).

(12) Anus is absent like coelenterates, with blind sac body plan.

(13) The nervous system is well developed. It is formed of longitudinal nerve cords with ganglia. A pair of anterior ganglia form the brain. The longitudinal nerve cords are connected together by transverse connectives.

(14) They are hermaphrodites, i.e., both male and female reproductive organs are present in the same animal.

(15) Fertilization is internal in them. Self or cross fertilization takes place in them.

(16) Their development is direct or indirect. Endoparasites show usually indirect development with many larval stages. Their life cycle is completed in one or two hosts.

(17) They are free living or parasitic. In parasitic worms adhesive organs like hooks, spines, suckers and adhesive secretions are present.

Classification of platyhelminthes : On the basis of digestive tract and free living or parasitic nature phylum platyhelminthes has been divided into three classes –

Class 1. Turbellaria (L. turbella, a string)

(1) Most of the turbellarians are free living but some of them are ectocommensal or parasitic, commonly called planarians or flat worms.

(2) The body epidermis is either cellular or syncytial and covered with cilia. Epidermis contains rhabdites.

(3) Segmentation is absent.

(4) Digestive system is present except in a few.

(5) Suckers are absent.

(6) Life cycle is simple, development direct.

Example : Dugesia, Notoplana, Bipalium, Thysanozoon, etc.

∙ Bipalium is only terrestrial planarian.

Class 2. Trematoda (Gr. trema, hole)

(1) Ecto or endoparasites of vertebrates commonly called flukes.

(2) Body mostly oval, unsegmented.

(3) Body wall without cilia, but covered by a thick, resistant, syncytial tegument.

(4) Suckers, and often hooks and spines, present for attachment to host tissues.

(5) Sense organs usually absent in adults.

(6) Digestive system well developed with terminal mouth, but no anus.

(7) Mostly hermaphrodite. Life cycle simple or complicated.

Examples : Polystomum, Fasciola, Schistosoma (blood fluke of man and other mammals), Opisthorchis, etc.

∙ Opisthorchis sinensis is commonly known as chinese liver fluke of man.

Class 3. Cestoda

(1) All endoparasites. Mostly in alimentary canal of vertebrates commonly called tapeworms.

(2) Body long and slender, tape-like, usually divided into small segments (= proglottids).

(3) Body wall non-ciliated, with a thick tegument.

(4) Anterior end with suckers and other attachment organs.

(5) No mouth, digestive system absent,਍igested liquid food is absorbed from host tissues by diffusion through body wall.

(6) Sense organs absent.

(7) Each proglottid contains one or two complete sets of hermaphrodite (bisexual) reproductive organs.

(8) Life-cycle usually complicated with alternation of hosts. Embryo hooked.

Examples – Taenia, Echinococcus, Hymenolepsis, Diphyllobothrium, Echinococcus, Dipylidium.

∙ Hymenolepsis is dwarf tapeworm. It is monogenetic tapeworm of man.

∙ Dipylidium is dog tapeworm.

∙ Diphyllobothrium is the largest tapeworms.

∙ Echinococcus is also called hydatid worm. Its hydatid cyst shows exogenous as well as endogenous budding. Parasite of small intestine of dogs, cats, etc. It has only 3-4 proglottids.

Phylum Aschelminthes (Nemathelminthes)-   The round worms

(Gk. nema = thread helmin = worm)

Brief History : Ancient people were familiar with certain large-sized nematode parasites of domestic animals. Minute nematodes were discovered only after the invention of microscope. Linnaeus (1758) included these in “Vermes” Rudophi (1793, 1819) included these under “Nematoidea” Gegenbaur (1859) ultimately proposed “Nemathelminthes” for these.

General Characters

(1) Many endoparasites of various animals and plants others free–living and widely distributed in all sorts of water and damp soil.

(2) Mostly minute or small some large (1 mm to 25 cm) some upto several meters long.

(3) Slender, cylindrical, elongated body usually tapering towards both ends, and unsegmented.

(4) Body wall formed of a thick, tough and shiny cuticle, a syncytial hypodermis beneath cuticle, and innermost layer of peculiar, large and longitudinally extended muscle cells arranged in four quadrants.

(5) Triploblastic, bilaterally symmetrical, pseudocoelomate, false coelom derived from embryonic blastocoel, unsegmented.

(6) Straight alimentary tract terminal mouth and anus. These are first animals to have complete gut.

(7) ‘Tube within a tube body’ plan, organ-system grade of body organization.

(8) Circulatory system and respiratory organs absent. A simple excretory system consists of protonephridia, comparatively simpler or complicated sensory organs, and a well developed nervous system present

(9) Reproductive system well developed. Usually unisexual with sexual dimorphism.

(10) Many kinds of Nematodes are parasites of useful plants and domestic animals. Some of these are pathogenic to their hosts, causing serious diseases. Even man is a host for more than 50 species, of which Ascaris lumbricoides and Enterobius vermicularis (pin worm) are very common. Other common human nematodes are Wuchereria which causes Filaria, Trichinella causing trichinosis, and Ancylostoma causing hookworm disease.

Classification of Nemathelminthes : On basis of the presence or absence of some specialized sense organs and caudal glands, and characteristics of excretory system, nematodes are classified into two classes –

Class 1. Phasmidia or Secernentea or Rhabditea

(1) Mostly parasitic.

(2) Possess a pair of unicellular, pouch-like sense organs, called phasmids, near hind end of body.

(3) Another pair of reduced, pore–like sense organs, called amphids, present near anterior end.

(4) Excretory system with paired lateral canals.

(5) Caudal glands absent.

Examples – Ascaris, Enterobius, Ancylostoma, Wuchereria, Trichuris, Trichinella, Dioctophyma, Rhabditis Necator, Gnathostoma, Dracunculus, Loa, etc.

Class 2. Aphasmidia or Adenophorea or Enoplea

(1) Mostly small, free-living.

(2) No phasmids.

(3) Amphids spiral, cord like or disc like, seldom pore like.

(4) No lateral excretory canals.

(5) Caudal glands present.

Examples : Enoplus, Dorylaimus, Mermis, Halichoanolaimus, Monohystera, Desmoscolex, etc.

Phylum Annelida – The segmented Animals

(L.annelus = ring, eidos = form)

Brief History : Linnaeus (1758) included all soft bodied worms in “Vermes”. Lamarck (1801) established phylum annelida for higher types of worms.

General characters

(1) Annelids are bilaterally symmetrical animals.

(2) They have organ-system grade of organization.

(3) They are coelomate (schizocoelomate) animals.

(4) They have triploblastic body wall.

(5) The muscle layers are thick in the body wall. Hence the body wall is said to be dermomuscular.

(6) The body is divided into a numerous segments called the metameres or somites. The segmentation is known as metamerism.

(7) The body is covered with a thin cuticle.

(8) Locomotory organs are setae.

(9) Digestive system is well developed. These have tube-within-a-tube body plan.

(10) Blood vascular system is a closed type

(11) Excretory system is formed of segmentally arranged nephridia.

(12) These always show cutaneous or skin respiration.

(13) Nervous system is formed of a pair of cerebral ganglia (brain) and a double ventral nerve cord.

(14) Mostly annelids are hermaphrodites. Fertilization is generally cross and may be external or internal.

(15) The gonoducts are formed from coelom (coelomoducts). The coelomoducts have connection with nephridia.

(16) Regeneration is common character in this phylum.

(17) Their development is direct or indirect and includes a free-swimming trochophore larva.

Classification of annelida : On the basis of position and arrangement of setae when present, absence and presence of sense organ, phylum annelida has been divided into four classes –

Class 1. Polychaeta (Gr. polus, many, chaite, hair)

(1) Polychaeta are marine and carnivorous.

(2) Body is elongated and segmented.

(3) Head consists of prostomium and peristomium and bear eyes, tentacles, cirri and palps, etc.

(4) Setae are numerous and are borne up on lateral prominances of the body wall known as parapodia.

(5) Locomotory organs are parapodia.

(6) Clitellum is absent.

(7) Cirri or branchiae or both may be present for respiration.

(8) Coelom is spacious usually divided by inter segmented septa.

(9) Alimentary canal is provided with an eversible buccal region and protrusible pharynx.

(10) Excretory organs are segmentally paired nephridia.

(11) Sexes are separate.

(12) Fertilization is external free swimming larval stage is trochophore.

(13) Asexual reproduction occurs by budding.

Examples : Nereis, Aphrodite, Polynae, Chaetopterus, Glycera, Arenicola, Amphitrite, Terebella, Sabella, Eunice, etc.

∙ Arenicola, Amphitrite and Terebella have external gills.

∙ Chaetopterus exhibits luminescence and great power of regeneration.

Class 2. Oligochaeta (Gr. oligi, few)

(1) They are mostly terrestrial or some fresh water forms.

(2) Body has conspicuous external and internal segmentation.

(3) Distinct head, eyes and tentacles are absent.

(4) Parapodia are absent.

(5) Locomotory organs are setae.

(6) Setae are usually arranged segmentally.

(7) Clitellum is usually present.

(8) Pharynx is not eversible and without jaws.

(9) They are hermaphrodites.

(10) Development is direct and takes place within cocoons secreted by clitellum.

(11) No free larval stage

Examples : Tubifex, Dero, Pheretima, (Indian earthworms), Lumbricus (European earthworm).

∙ Tubifex and Dero are fresh water forms. Tubifex can live in polluted water where oxygen availability is poor because its has a large amount of haemoglobin in blood.

Class 3. Hirudinea (L. hirudo, a leech)

(1) This class includes mostly ectoparasitic and fresh water forms, while few are marine, feeding upon fishes and other animals.

(2) Body is elongated usually flattened dorso–ventrally or cylindrical.

(3) Body consists or definite number of segments, each segments breaks up into 2 to 4 rings or annuli.

(4) Parapodia and setae are absent.

(5) Body is provided with an anterior and a posterior sucker, both situated ventrally.

(6) Coelom is reduced by botryoidal tissue.

(7) Mouth opens on the ventral surface in the anterior sucker, while anus opens dorsal to the posterior sucker.

(8) Locomotory organs are suckers.

(9) Hermaphrodite i.e., sexes united.

(10) Reproduction sexual. Asexual reproduction is unknown.

(11) Eggs are usually laid in cocoons.

(12) Development is direct without free swimming larval stage.

Examples : Acantobdella, Glossiphonia (Fresh water leeches), pontobdella, Haemodipsa, etc.

∙ Haemodipsa is terrestrial leech.

∙ Pontobdella is a ectoparasite on elasmobranchi fishes.

∙ Acanthobdella is a ectoparasite of salmon fish.

Class 4. Archiannelida (Gr. archi, primitive)

(1) They are exclusively marine forms.

(2) Body elongated and worm-like.

(3) Setae and parapodia are usually absent.

(4) External segmentation is slightly marked by faint while internal segmentation is marked by coelomic septa.

(5) Prostomium bears two or three tentacles.

(6) Unisexual or hermaphrodite.

(7) Larva is typical trochopore.

Examples : Polygordius, Protodrillus, Nerilla, Saccocirrus, etc.

∙ Polygordius is a primitive Archiannelid or living fossil.

Phylum Arthropoda – The animals with jointed feet

(Gk. Arthron = joint Podos = foot)

Brief History : Aristotle described a few crabs and other arthropods. Linnaeus included all such animals in his group “Insecta”. Lamarck divided this group into three classes – Crustacea, Hexapoda and Arachnida. Finally, Von seibold (1845) established the phylum Arthropoda for these animals.

General characters

(1) Occur widely on land, in air, and in all sorts of water, from snowy tops of high mountains to the depths of ocean. Many are parasites of other animals and plants. Hence, the phylum is of great economic importance.

(2) Bilateral, triploblastic, body segmented and also divided into head, thorax and abdomen. Segmentation marked only externally number of segments or somites fixed and each has its separate exoskeleton of thick and hard, chitinous cuticle secreted by epidermis of body wall. Head somites always fused.

(3) Each segment basically bears a pair of lateral jointed appendages adapted for food ingestion, locomotion, respiration, copulation, etc.

(4) Muscular system well-developed muscle fibres always striated.

(5) Digestive tract complete. Most head appendages from mouth parts with lateral jaws for chewing or sucking. Anus terminal.

(6) Coelom reduced to small cavities in excretory and reproductive organs replaced elsewhere by blood sinuses which merge together to form a large perivisceral cavity – the haemocoel - around viscera. Sinuses form an “open blood vascular system” filled with haemolymph which may contain haemocyanin. Haemocoel communicates with a long tubular and pulsatile, mid-dorsal heart.

(7) Respiration by gills (aquatic forms), or trachea or book lungs (terrestrial forms) by diffusion through body surface in some.

(8) Excretion by coelomoducts or specialized green or coxal glands, or by malpighian tubules. Excretory product is uric acid.

(9) Nervous system basically similar to the typical annelid plan head with a brain-ring which is connected to a double ventral nerve cord, having paired segmental ganglia which represent true metamerism. Well developed sensory organs of various types.

(10) Cilia completely absent. Muscles mostly striated and capable of rapid contraction.

(11) Sexes mostly separate with sexual dimorphism. Paired reproductive organs and ducts.

(12) Fertilization typically internal, in female&aposs body. Eggs megalecithal. Oviparous or viviparous

(13) Life cycle includes one or more larval stages that metamorphose into adults.

Classification of Arthropoda : On the basis of body shape, degree of segmentation and regionation, and presence or absence of certain appendages (antennae, mandibles and chelicerae), phylum Arthropods is divided into four subphyla Biggest phylum in regard to the number of species is Arthropoda.

Subphylum (I) Onychophora (Gr. Oychos = claw, phoros = bearing)

(1) Terrestrial walking worms.

(2) Body cylindrical with indistinct extenral segmentation.

(3) Unjointed 14-43 pairs of legs.

(4) Head not distinct, Oviparous/Viviparous

(5) A pair of eyes, short antennae and blunt oral papillae.

(6) Excretory organs are metanephridia segmentally arranged.

(7) A living connecting link forming a transitional link between Annelida and Arthropoda.

Example : Peripatus, Ophisthopatus, Ooperipatus, etc.

Subphylum (II) Trilobitomorpha (Gr. TRIA = Three LOBOS = lobe MORPHE = form)

(1) Most primitive, extinct, marine arthropods of Cambrian to Permian rocks.

(2) 10 to 675 mm. Long body covered by a hard segmented shell distinct head of four fused somites bearing a pair of antennae, four pairs of appendages and often a pair of eyes.

(3) Trunk divided, by two longitudinal furrows, into 3 lobes.

(4) Abdominal region of 2 to 29 somites and a fused caudal plate or pygidium.

(5) Each segment, except the last one, bears a pair of biramous jointed appendages.

Example – Triarthrus, Dalmenites.

Subphylum (III) Chelicerata (Gr. CHELA = Claw CEROS = Horn ATA = Group) 

(1) Mostly terrestrial, free–living and small–sized.

(2) Body distinguished into head, thorax and abdomen (= opisthosoma). Head and thorax fused to form a cephalothorax or prosoma.

(3) Cephalothorax with eyes and six pairs of appendages – One pair of clawed and jointed chelicerae in place of mandibles, one pair of pedipalps, and four pairs of walking legs. Antennae absent. Abdomen with or without appendages, but distinguished into a large and broader mesosoma, a small metasoma and a long and narrow, tail–like telson.

(4) Respiration by gills book–lungs or tracheae.

(5) Excretion by malpighian tubules or coxal glands, or both.

(6) Sexes mostly separate females oviparous development direct or through a larval stage. Divided into three classes on the basis of respiratory organs

Class 1. Merostoma

(1) They are Marine.

(2) Respiration by gills.

(3) Cephalothorax with lateral compound eyes median simple eyes and six pairs of usual appendages.

(4) Abdomen with 5 to 6 pairs of gill-bearing appendages.

(5) Hind end forms a long bayonet–like telson.

Example– Limulus (The king-crab).

∙ Limulus is a living fossil.

Class 2. Arachnida

(1) Mostly terrestrial spiders, scorpions, mites, ticks, etc.

(2) Respiration by book–lungs or trachea.

(3) Eyes simple.

(4) Abdomen without appendages.

(5) Many with poison glands and poison fangs, jaws of stings.

(6) No gills.

(7) Life-cycle without metamorphosis.

Examples– Palamnaeus (scorpion), Lycosa, mites, ticks.

∙ Lycosa is a common web–spinning spider web–spinning glands are situated in posterior part of abdomen. 

Class 3. Pycnogonida or Pentapoda

(1) Small–sized marine sea–spiders.

(2) Cephalothorax 3–segmented forms major part of body abdomen vestigial.

(3) Suctorial mouth on top of a long proboscis.

(4) Head usually with 4 pairs of appendages and 4 eyes.

(5) 5, 6 or 12 pairs of long walking legs.

(6) No special respiratory and excretory organs.

(7) Unisexual females oviparous. Eggs carried by males.

Example – Nymphon.

Subphylum (IV) Mandibulata or Antennata (L.MANDIBULA = Mandible ATA =group)

(1) Body divided into head and trunk, or head, thorax and abdomen.

(2) Segmentation distinct.

(3) 1 or 2 pairs of antennae, 1 pair of mandibles in place of chelicerae, one or more pairs of maxillae and 3 or more pairs of walking legs.

(4) Eyes mostly compound.

(5) Respiration by gills or trachea.

(6) Excretion by malpighian tubules or antennal glands.

(7) Unisexual life cycle usually with larval forms. Divided into six classes.

Class 1. Crustacea

(1) Mostly aquatic.

(2) Body divided into cephalothroax and abdomen.

(3) Dorsally, cephalothorax covered by a thick exoskeletal carapace.

(4) Head of 5 segments, with 2 pairs of antennae, one pair of mandibles and 2 pairs of maxillae thorax of 2 to 60 distinct or variously fused somites abdominal somites usually distinct with a posterior telson.

(5) Appendages mostly biramous.

(6) Respiration through body surface or by gills.

(7) Excretion by special coxal glands in antennae or maxillae.

(8) Mostly unisexual genital ducts and pores paired females oviparous.

(9) Life-cycle usually with larval forms.

Examples – Palaemon, Cancer, Cyclops, Astacus, Sacculina, Cypris, Daphnia, etc.

∙ Tiny crustaceans such as Daphnia and cyclops act as zooplankton which form important link in the food chain in water.

Class 2. Insecta

(1) Aquatic, terrestrial or aerial.

(2) Body divided into head, thorax and abdomen.

(3) Segments 6 in head, 3 in thorax and 11 or less in abdomen.

(4) Legs typically 3 pairs (Hexapoda) aerial forms with one or two pairs of wings.

(5) Head with 1 pair of large, compound eyes, 1 pair of antennae and variously modified mouth–parts.

(6) Respiration by branched tracheae.

(7) Excretion by specialized malpighian tubules.

(8) Unisexual females oviparous.

(9) Life-cycle simple or complicated.

Examples – Periplaneta, Musca, Mosquitoes, locusts, butterflies, bees, wasps, termites, silverfish, beetles, etc.

∙ Insects are of great economic importance to mankind.

∙ Silverfish is not a fish. 

Class 3. Diplopoda

(1) Terrestrial.

(2) Body long, cylindrical, worm–like.

(3) 5-segmented head with 1 pair each of short antennae, mandibles and maxillae 2 groups of simple eyes.

(4) Thorax of 4 segments, each except the first with a pair of joined legs.

(5) Abdomen of 9 to 100 or more segments, but each apparent segment formed by fusion of two and, hence, bears 2 pairs of legs, spiracles, ostia and nerve ganglia.

(6) Respiration by tracheae.

(7) Excretion by malpighian tubules.

(8) Unisexual gonad single females oviparous.

Example – Thyroglutus (millipede)

∙ Thyroglutus damages the root of crop plants.

Class 4. Chilopods

(1) Terrestrial.

(2) Body long, worm–like, somewhat dorso–ventrally flattened and divided into head and trunk.

(3) Segments 15 to 181 not fused in pairs each with a single pair of legs first pair of legs claw like and each contains a poison gland.

(4) Head with a air each of long antennae and mandibles, and 2 pairs of maxillae.

(5) Respiration by tracheae.

(6) Unisexual females oviparous or viviparous. Genital openings mid ventral on last but one segment.

(7) Excretion by malpighian tubules.

Example – Scolopendra (centipede).

∙ Diplopoda and chilopoda are together kept in myriapoda.

Class 5. Symphyla

(1) Terrestrial.

(2) Body upto 6 mm. Long divided into head and trunk.

(3) Head like that of insects, but without eyes.

(4) Trunk of 15 to 22 somites bears 10 to 12 pairs of legs.

(5) Genital pores mid ventral between legs of 4th pair.

Example – Scutigerella (the garden centipede).

Class 6. Pauropoda

(1) Terrestrial.

(2) Minute, soft and cylindrical, worm like body divisible into head and trunk.

(3) Head with one pair each of branched antennae and unbranched mandibles and maxillae no eyes.

(4) Trunk of 11 or 12 somites which are dorsally fused in pairs.

(5) Legs 9 to 10 pairs.

(6) Genital pores ventral on 3^rd trunk segment.

Example – Pauropus.

Phylum Mollusca – The soft bodied animals

(L., Mollis or Molluscus = Soft bodied)

Brief History : Aristotle described a number of molluscs. Johnston (1650) proposed the name of the phylum.

General characters

(1) Molluscs are multicellular organisms.

(2) They are mostly marine.

(3) They have a bilateral symmetry, but snails are asymmetrical.

(4) They are triploblastic animals.

(5) They are coelomate animals. True coelom is reduced the haemocoel is well developed in them.

(6) They have organ system grade of organization.

(7) The body is soft and unsegmented.

(8) The soft body is covered by a fleshy fold of the body wall. It is called mantle.

(9) The molluscs are provided with one or two calcareous shells. The shells may be external or internal, univalve or bivalve.

(10) Respiration is carried out by the gills or pulmonary chambers.

(11) The digestive system is well developed. It contains a radula and a hepatopancreas.

(12) The circulatory system is of an open type. Blood with amoebocytes, respiratory pigment is copper containing haemocyanin dissolved in plasma.

(13) The excretory organ is the kidney (organ of Bojanus).

(14) The nervous system is well developed with paired ganglia, commissures and connectives.

(15) The sensory organs are eyes, statocysts and osphradia (a chemoreceptor to test chemical nature of water).

(16) Reproduction sexual. Sexes are separate in them, or they are hermaphrodites.

(17) The development in their case is either direct or indirect with free larval forms like trochophore, veliger, glochidium, etc.

Classification of Mollusca : On the basis of body shape and symmetry and characteristics of foot mantle, respiratory organs, nervous system, etc. phylum mollusca are divided into seven classes :

Class 1. Monoplacophora

(1) The body is bilaterally symmetrical and segmented.

(2) The shell is formed of a single valve.

(3) The head is without eyes and tentacles.

(4) The gills are external and serially arranged.

(5) The nephridia are five pairs.

Example : Neopilina galathea

∙ Neopilina is a living fossil and connecting link between Annelida and Mollusca.

Class 2. Aplacophora or Solenogasters

(1) The body is worm–like, bilaterally symmetrical and cylindrical.

(2) The head, mantle, foot, shell and nephridia are absent.

(3) The body is covered with spicule-bearing cuticle.

(4) The digestive tract is straight with radula.

(5) A mid dorsal longitudinal keel or crest is often present .

Example : Neomenia, Chaetoderma, etc.

Class 3. Polyplacophora

(1) These molluscs are bilaterally symmetrical, and dorsoventrally flattened.

(2) Head small, without eyes and tentacles.

(3) The shell is composed of a longitudinal series of 8 plates.

(4) The foot is flat and ventral.

(5) The radula is well developed.

(6) Respiration by 8 to 60 pairs of gills.

(7) Unisexual only one gonad trochophore larval stage.

Example : Chiton, Cryptochiton, etc.

∙ On the dorsal surface of chiton is a convex shell composed of 8 transversely elongated calcareous plates arranged in a longitudinal manner.

Class 4. Gastropoda

(1) It is the largest class of Mollusca.

(2) It seems that these animals are moving on their stomach. Hence the name gastropoda.

(3) Gastropods are marine, fresh water or terrestrial animals. A few are parasitic.

(4) The body is unsegmented and asymmetrical.

(5) The shell is univalve and spirally coiled due to torsion.

(6) The head is distinct. It bears tentacles, eyes and a mouth.

(7) The foot is ventral and muscular.

(8) The buccal cavity is provided with a radula.

(9) The circulatory system is open.

(10) Respiratory organs gills (ctenidia), or pulmonary sac or both.

(11) Nervous system usually with four pairs of ganglia.

(12) The sexes are mostly separate, while some forms are hermaphrodite.

(13) The development includes veliger and trochophore larvae.

Examples : Haliotis, Cypraea, Pila, Murex, Aplysea, Doris, Limax, Patella, etc.

∙ Limax is a terrestrial gastropod. It creeps on a self-secreted tract.

∙ Doris is a marine gastropod, commonly called true limpet. It has an aspidobranch gill.  

Class 5. Scaphopoda

(1) It is the small group of marine molluscs.

(2) The foot is boat–shaped.

(3) The eyes, the tentacles and ctenidia are absent.

(4) Marine, bilaterally symmetrical molluscs.

Examples : Siphonodentalium Dentalium, and Pulsellum

∙ Dentalium is commonly called tusk shells.

Class 6. Pelecypoda or Bivalvia or Lamellibranchiata

(1) Pelecypoda are aquatic in habit.

(2) The body is bilaterally symmetrical and laterally compressed.

(3) The shell is formed of two distinctive shell plates.

(4) The head is not distinct.

(5) The alimentary canal shows a crystalline style.

(6) The gills, excretory organs and the other structures are paired.

(7) The sexes are separate or united.

(8) The development is indirect having a glochidium larva.

Example : Mytilus, Unio, Teredo, Lamellidens, Solen, Pecten, Punctada, etc.

∙ Teredo bores through wood of ship but is without segmentation.

∙ Pearl oysters belongs to the class pelecypoda.

Class 7. Cephalopoda or Siphonopoda

(1) Most developed, marine and actively swimming by ejecting jets of water through exhalent siphon of mantle.

(2) The body is bilaterally symmetrical.

(3) The foot is modified into arms and funnel.

(4) The shell may be either absent or rudimentary it may be internal or external.

(5) Nervous system is highly developed.

(6) The odonotophore with a radula is present.

(7) The ink–gland is present.

(8) The sexes are separate.

(9) The development is direct hence no metamorphosis and larval stage.

Example : Nautilus, Loligo Sepia, Octopus, etc.

∙ Nautilus has an external coiled and chambered shell.

∙ In octopus has good learning power and can be trained.

∙ Members of genus Architeuth is are known as giant squid and are largest living invertebrates.

Phylum Echinodermata – The spiny skinned animals

(Gk. echinos = spines derma = skin/covering)

Brief History : Although Jacob Klein (1738) had earlier coined the name "Echinodermata”, yet Linnaeus included these animals under “Mollusca”, and Lamarck under his class “Radiata” as "Echinodermes”. Finally, Leuckart (1847) raised the group to the status of a separate phylum.

General characters

(1) Echinoderms are exclusively marine beings.

(2) They are triplobalstic and coelomate (enterocoetomate) animals.

(3) They have radially symmetrical body. The radial symmetry is due to sedentary or sessile mode of life and it is a secondary character in echinoderms.

(4) They have organ system grade of organization.

(5) They have well developed endoskeleton formed of calcareous ossicles and spines.

(6) They have a water–vascular system (Ambulacral system) with tube-feet for locomotion, feeding and respiration.

(7) Circulatory system is of the open–type.

(8) Respiratory organs include dermal branchiae, tube feet, respiratory tree and bursae.

(9) Nervous system is complex and contains both central and peripheral components, but no brain.

(10) The sensory organs are poorly developed.

(11) The excretory organs are absent.

(12) They have pedicellariae.

(13) Development is indirect.

(14) The larval forms are bilaterally symmetrical.

(15) Regeneration power is well developed in Echinoderms.

Classification of Echinodermata : On the basis of body shape, position of madreporite and kind of larval form, echinoderms are classified into two subphylum.

Subphylum (I) Eleutherozoa ­: Free-living echinoderms with ventral mouth.

Class 1. Asteroidea

(1) Starfishes or sea stars.

(2) Arms 5 or more and not sharply marked off from the central disc.

(3) Tube feet in orally placed ambulacral grooves with suckers.

(4) Anus and madreporite aboral.

(5) Pedicellariae present.

(6) Free-living, slow-creeping, predaceous and scavengerous.

Examples : Astropecten, Luidia, Goniaster, Oreaster (= Pentaceros), Asterina, Solaster, Pteraster, Echinaster, Asterias, Heliaster, etc.

Class 2. Ophiuroidea

(1) Brittle-stars and allies.

(2) Body star-like with arms sharply marked off from the central disc.

(3) Pedicellariae absent.

(4) Stomach sac-like no anus.

(5) Ambulacral grooves absent or covered by ossicles tube feet without suckers.

(6) Madreporite oral.

Examples : Ophiura, Ophiothrix, Ophioderma, Ophiopholis, Gorgonocephalus, Asteronyx.

Class 3. Echinoidea

(1) Body not divided into arms globular (sea urchins), or flattened disc-like (sea-cakes).

(2) Mouth at lower pole, covered by 5 strong and sharp teeth, forming a biting and chewing apparatus called 𠇊ristotle&aposs Lantern”.

(3) Tube-feet slender with suckers.

(4) Skin ossicles fused to form a rigid globular, disc like, or heart-shaped shell or test with movable spines.

(5) 3–jawed pedicellariae present in skin.

(6) Gut long, slenderical and coiled. Anus present.

(7) Larval forms pluteus and Echinopluteus.

Examples : Echinus, Clypeaster, Echinarachinus, Echinocardium, etc.

∙ Members of Echinoidea are also known as Floating stone.

Class 4. Holothuroidea

(1) Body massive, long and cylindrical like a cucumber elongated in oral-oboral axis no arms.

(2) Mouth at anterior and anus at posterior ends.

(3) Mouth surrounded by many hollow retractile tentacles.

(4) Tube feet usually present sucker-like.

(5) Skin leathery, but relatively soft, without spines or pedicellariae may have an endoskeleton of minute calcareous ossicles.

(6) Respiration and excretion by two long and highly branched tubes (= respiratory tree) extending into coelom from cloaca.

(7) Larval form Auricularia.

Examples – Holothuria, Cucumaria etc.

Subphylum (II) Pelmatozoa : Stalked, sedentary echinoderms, with mouth situated on upper side.

Class 1. Crinoidea

(1) Body flattened and pentamerous distinguished into a small and circular central disc and five or more (in multiples of five) long, then, branched and flexible arms radiating from the disc.

(2) Disc enclosed in a hard, cup–shaped calyx formed of calcareous plates calyx attached to a substratum by a stalk or simply by its aboral surface.

(3) Mouth in middle and anus excentral upon a cone, both upon oral surface. 5 ambulacral grooves run from mouth upto the tips of the arms.

(4) Tube feet sucker–like restricted to central disc can help in food-collection.

(5) Some forms (sea–lilies) permanently sessile and attached to sea-bottom by a long stalk others (feather stars) free-swimming, but have flexible cirri for gripping objects in water.

(6) Spines and pedicellariae absent in skin.

Examples : Antedon, Neometra, etc.