Current Affairs 11th Class

Category : 11th Class

 (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.

Common Names