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Phylum Mollusca and Echinodermata

Phylum Mollusca

(i) Basically bilateral and protostomial eucoelomate eumetazoans whose soft body (L., mollis or molluscum = soft)

is non-segmented and enclosed within a skin–fold (mantle) which usually secretes a calcareous shell.

Salient Features

(1) Molluscs are multicellular organisms

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

(3) They are triploblastic animals.

(4) They are coelomate animals.

(5) They have organ system grade of organization.

(6) The body is soft and non-segmented.

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

(8) The molluscs are provided with one or two calcareous shells.

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

(10) The digestive system is well developed.

(11) The circulatory system is of an open type.

(12) The excretory organ is the kidney.

(13) The nervous system is well developed.

(14) The sensory organs are eyes, statocysts and osphradia.

(15) Sexes are separate in them, or they are hermaphrodites.

(16) The development in their case in either direct or indirect

Classification of Mollusca

Class 1 - Aplacophora or Solenogasters

  • The body is worm–like, bilaterally symmetrical and cylindrical.

  • The head, mantle, foot, shell and nephridia are absent.

  • The body is covered with spicule–bearing cuticle.

  • The digestive tract is straight with radula.

  • A mid dorsal longitudinal keel or crest is often present.

  • Example: Neomenia, Chaetoderma, etc.

Class 2 - Monoplacophora

  • The body is bilaterally symmetrical and segmented.

  • The shell is formed of a single valve.

  • The head is without eyes and tentacles.

  • The gills are external and serially arranged.

  • The nephridia are five pairs.

  • Example: Neopilina galatheae

Class 3 - Polyplacophora

  • These molluscs are bilaterally symmetrical, and dorsoventrally flattened.

  • The shell is composed of a longitudinal series of 8 plates.

  • The foot is flat and ventral.

  • The radula is well developed.

  • Example: Chiton, Cryptochiton, etc.

Class 4 - Gastropoda

  • It seems that these animals are moving on their stomach. Hence, the name is gastropoda.

  • Gastropods are marine, fresh water or terrestrial animals. A few are parasitic.

  • The body is non-segmented and asymmetrical.

  • The shell is univalve and spirally coiled.

  • The head is distinct. It bears tentacles, eyes and a mouth.

  • The foot is ventral and muscular.

  • The buccal cavity is provided with a radula.

  • The circulatory system is open.

  • The sexes are mostly separate, while some forms are hermaphrodite.

  • The development includes veliger and trochophore larvae.

  • Examples: Haliotis, Cypraea (Cowrie) Pila (apple snail)

Class 5 - Scaphopoda

  • The foot is boat–shaped.

  • The eyes, the tentacles and ctenidia are absent.

  • Marine, bilaterally symmetrical molluscs.

  • Examples: Dentalium, Siphonodentalium and Pulsellum

Class 6 - Pelecypoda

  • Pelecypoda are aquatic in habit.

  • The body is bilaterally symmetrical and laterally compressed.

  • The shell is formed of two distinctive shell plates.

  • The head is not distinct.

  • The alimentary canal shows a crystalline style.

  • The gills, excretory organs and the other structures are paired.

  • The sexes are separate or united.

  • The development is indirect having a glochidium larva.

Class 7 - Cephalopoda

  • The body is bilaterally symmetrical.

  • The foot is modified into arms and funnel.

  • The shell may be either absent or rudimentary

  • The odonotophore with a radula is present.

  • The ink–gland is present.

  • The sexes are separate.

  • The development is direct hence no metamorphosis and larval stage.

  • Example: Nautilus, Loligo Sepia, Octopus

Phylum Echinodermata

(i) The term “Echinodermata” means spiny skin (Gr., echinos = spiny + dermatos = skin).

Salient features

(1) Echinoderms are exclusively marine beings.

(2) They are triplobalstic and coelomate animals.

(3) They have radially symmetrical body.

(4) They have organ system grade of organization.

(5) They have well developed

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

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

(8) The sensory organs are poorly developed.

(9) The excretory organs are absent.

(10) They have pedicellariae.

(11) Development is indirect.

(12) The larval forms are bilaterally symmetrical.

Classification of Echinodermata

Subphylum I - Eleutherozoa ­: Free-living echinoderms

Class 1 - Asteroidea

  • Starfishes or sea stars.

  • Arms 5 or more and not sharply marked off from the central disc.

  • Tube feet in orally placed ambulacral grooves; with suckers.

  • Anus and madreporite aboral.

  • Pedicellariae present.

  • Free-living, slow-creeping, predaceous and scavengerous.

Class 2 - Ophiuroidea

  • Brittle-stars and allies.

  • Body star-like with arms sharply marked off from the central disc.

  • Pedicellariae absent.

  •  Stomach sac-like; no anus.

  • Ambulacral grooves absent or covered by ossicles; tube feet without suckers.

  • Madreporite oral.

Class 3 - Echinoidea

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

  • Mouth at lower pole, covered by 5 strong and sharp teeth, forming a biting and chewing apparatus called “Aristotle's Lantern”.

  • Tube-feet slender with suckers.

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

  • 3–jawed pedicellariae present in skin.

  • Gut long, slender and coiled.

  • Larval forms pluteus and Echinopluteus.

  • Examples – Sea urchins and sand dollars etc.

Class 4 - Holothuroidea

  • Body massive, long and cylindrical like a cucumber

  • Mouth at anterior and anus at posterior ends.

  • Mouth surrounded by many hollow retractile tentacles.

  • Tube feet usually present; sucker-like.

  • Skin leathery, but relatively soft, without spines or pedicellariae; may have an endoskeleton of minute calcareous ossicles.

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

  • Larval form Auricularia.

  • Examples – Holothuria, Cucumaria etc.

  • Subphylum II - Pelmatozoa: Stalked, sedentary echinoderms.

Class 5 - Crinoidea

  • Body flattened and pentamerous.

  • Disc enclosed in a hard, cup–shaped calyx formed of calcareous plates.

  • Mouth in middle and anus excentral upon a cone, both upon oral surface.

  • Tube feet sucker–like; restricted to central disc; can help in food–collection.

  • Some forms (sea–lilies) permanently sessile and attached to sea–bottom by a long stalk; others (feather stars) free-swimming.

  • Spines and pedicellariae absent in skin.

  • Examples – Sea lilies and Feather stars (Antedon)

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