One of the most pressing Biology Topics of our time is the conservation of endangered species and habitats.
Phylum Mollusca – Characteristics, Classification
Phylum Mollusca (L., molluscus – soft)
1. Body is soft, bilaterally symmetrical, with little segmentation and without appendages. The size of the body varies from a microscopic to a giant form such as an Octopus of up to 50 feet.
2. Body is divisible into an anterior head, a ventral muscular foot, and a hard dorsal visceral mass. The entire body is covered by a fold of thin skin, called mantle which secretes a hard calcareous shell of one or more pieces. The foot is mainly used in locomotion.
3. Body cavity is haemocoel. The true coelom is reduced and restricted to the pericardial cavity and the lumen of the gonads and nephridia.
4. Digestive tract has a simple structure.
5. Respiration through gills (called ctenidia), mantle, or a “lung” of the mantle.
6. Circulatory system is open except in cephalopods.
7. Excretion by a pair of metanephridia or kidneys.
8. Sexes are usually separate.
9. Sensory organs of touch, smell, taste, equilibrium, and vision (in some).
10. Aquatic, mostly marine, fresh water, and some terrestrial forms.
Osmoregulatory organs known as nephridia occur in many invertebrates. When they are closed at the inner end, they are called protonephridia, but when they open into a coelomic space at their inner end, they are called metanephridia.
The phylum Mollusca is divided into the following five main classes:
Class 1. Polyplacophora – Shell composed of eight calcareous pieces.
Class 2. Gastropoda – Univalve shell and visceral mass are spirally coiled (torsion).
Examples: Pila (apple snail), Aplysia (sea hare), Helix (garden snail), and Umax (gray slug).
Class 3. Scaphopoda – Tusk-like tubular shell opens at both ends.
Examples: Dentalium (tusk shell)
Class 4. Pelecypoda (Bivalvia) – Shell consists of two lateral valves hinged together mid-dorsally.
Examples: Unio (freshwater mussel), Mytilus (sea mussel); Pinctada (Indian pearl oyster), Teredo (shipworm).
Class 5. Cephalopoda – Shell external, internal or absent, Foot altered into a series of sucker-bearing arms or tentacles, Eyes are highly developed.
Examples: Nautilus, Loligo (squid), Sepia (cuttle-fish), Octopus (devil-fish).