Phylum Mollusca : General Characteristics
(L., mollis=soft)
Second largest phylum in number ; first being arthropoda
Malacology : Study of Mollusca
Conchology : Study of shell of Mollusca
Most molluscs are marine, some freshwater, some terrestrial.
Most molluscs are herbivorous but some (e.g. squid) are active predators.
Triploblastic, soft-bodied animals with bilateral symmetry except gastropods which are asymmetrical due to twisting known as torsion.
Body is unsegmented except Neoplina
Pricipal body cavity is haemocoel; i.e. haemocoelomate
Circulatory system is open except in cuttlefish.
Body is covered by a skin fold mantle with shell gland which secrete calcareous spicules, shell plates or shells.
Body wall is single layered and consists of ciliated epidermis.
Body is organized into three general regions; head, foot (ventral) and visceral hump. Head bears mouth and sense organs,; foot is used for locomotion and capturing prey ; visceral hump contains digestive tract and visceral organs.
Digestive system is complete along with presence of a digestive gland liver
Buccal cavity has a rasping organ called radulla which is armed with transverse rows of chitinous teeth. Each row contains 7 teeth.
Blood with amoebocytes, respiratory pigment is copper containing haemocyanin (blue color blood) dissolved in plasma.
Respiratory organs includes several comb like gills or ctenidia(in aquatic) and pulmonary sacs or lungs (in aerial) ; sometimes direct.
Excretion by a pair of kidneys/ organ of Bojanus and pericardial gland or kebers gland.
Nervous system with paired ganglia, commissures and connectives.
Sense organs include eyes, statocysts, tentacles, osphradia, lithocysts.
Reproduction sexual ; dioecious or monoecious.
Fertilization is generally external ; mostly oviparous , few viviparous.
