General Characteristics of Phylum Mollusca

Mollusca (Molluscus- soft bodies)

Study of Mollusca is called Malacology.
The term Mollusca was first applied by Aristotle to cuttle fish.
Lamarck coined the term Molluscus.
Study of molluscan shell is called conchology.

The animals of this phylum are soft-bodied, non-metameric, triploblastic coelomates and fundamentally bilaterally symmetrical invertebrates with a thin fleshy envelope the mantle, around the visceral (internal organs of the body) which may secrete calcareous shell.
They are sluggish animals.

General characters

1. Widely distributed over the world. Mostly aquatic (marine and few are fresh-water) and some are terrestrial living in damp soil.
2. Body is soft and unsegmented (except Neopilina).
3. Bilaterally symmetrical except gastropodas (asymmetrical) because the shell is coiled.
4. Schizocoel or Haemocoel cavity found as coelom.
5. Triploblastic
6. Organ system level of body organization.
7. Body is divisible into head (absence in pelecypoda and scaphopoda), dorsal visceral mass, ventral muscular foot and mantle.
   Mantle- is a dorsal glandular fold of the body wall. It is thick and muscular and encloses a mantle cavity.
   Head- anterior part of the body, which contains mouth, eyes and tentacles and other sense organs.
   Foot-is ventral in position usually thick and muscular and generally forms the main locomotory organs.
   Visceral mass- on dorsal side contains digestive, circulatory and reproductive organ of the body and it forms a hump or dome.
8. Body is covered by a hard calcareous structure, made up of calcium carbonate, called shell. In some molluscus, it is internal or reduced or even absent (oplacophora).
9. Locomotion takes place by ventral muscular foot.
10. Digestive system is well developed with a hard chitinous structure, called radula.
11. Respiration takes place through one or more gills or ctenidia, lungs (pulmonary sac) or general body surface in the terrestrial forms.
12. Circulatory system is closed or open type.
13. Head consists tentacles and compound eyes.
    Presence of one pair tentacles except octopus where tentacles are modified into arm.
14. Excretion takes place by paired metanephridia (kidney).
    
15. Nervous system consists of many paired ganglia, connectives and nerves.
16. Sense organs are eyes, tentacles, osphradium and statocyst.
17. Sexes are usually separate but some are monoecious.
    Pila- sexual dimorphism.
    Limax- hermaphrodite
    Helix- hermaphrodite
18. Fertilization is external (in sessile group such as in oyster) or internal.
19. Development may be direct or indirect. Larva is trochopore or veliger or parasitic stage called glochidium larva.

Examples

Things to remember

1. Neopilina is considered as connecting link between Annelida and Mollusca. It is primitive with segmented body.
2. 2^nd largest animal phylum after Arthropoda.
3. Unio contains a digestive gland called Green gland, which is comparable to liver of vertebrates.
4. Statocysts- for maintaining equilibrium of the body.
5. Osphradia- chemoreceptor to test chemical nature of the water.
6. Excretion by a pair of kidneys or organs of Bojanus and pericardial gland or Keber’s organ.
7. Octopus is known as devil fish. It is a cephalopod having 8 arms and without a shell.
8. Heart is myogenic.
9. Ink glands are present in cephalopoda that are concerned with protection.
10. Blood consists of copper containing pigment called haemocyanin. So blood color is blue or green.

General Characteristics of Phylum Mollusca

Tags: Examples of Mollusca, General characters of Mollusca, Mollusca, Molluscans, Neopilina as connecting link, Osphradium, Statocyst