⚫ 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:
- Molluscs are multicellular organisms
- They have a bilateral symmetry, but snails are asymmetrical
- They are triploblastic animals.
- They are coelomate animals.
- They have organ system grade of organization.
- The body is soft and non-segmented.
- The soft body is covered by a fleshy fold of the body wall. It is called mantle.
- The molluscs are provided with one or two calcareous shells.
- Respiration is carried out by the gills or pulmonary chambers.
- The digestive system is well developed.
- The circulatory system is of an open type.
- The excretory organ is the kidney.
- The nervous system is well developed.
- The sensory organs are eyes, statocysts and osphradia.
- Sexes are separate in them, or they are hermaphrodites.
- 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
⚫ The term “Echinodermata” means spiny skin (Gr., echinos = spiny + dermatos = skin).
⚫ Salient features:
- Echinoderms are exclusively marine beings.
- They are triplobalstic and coelomate animals.
- They have radially symmetrical body.
- They have organ system grade of organization.
- They have well developed
- They have a water–vascular system with tube–feet for locomotion, feeding and respiration.
- Circulatory system is of the open–type.
- The sensory organs are poorly developed.
- The excretory organs are absent.
- They have pedicellariae.
- Development is indirect.
- 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)