The document provides an introduction and overview of the general characteristics of molluscs. It states that molluscs are soft-bodied animals that inhabit all habitats except air, have existed since the early Cambrian period, and are the second largest phylum of animals. The general characteristics described include having a triploblastic, bilaterally symmetrical body divisible into head, mantle, foot, and visceral mass, with shells sometimes present either internally or externally. Digestion and respiration occur through various organs like the radula, gills, lungs, and the circulatory system utilizes haemocyanin.

2. INTRODUCTION
 The term Mollusca was coined by Aristotle
 The term Mollusca was defined by Cuvier
 Soft bodied animals.
 Second largest phylum
 Malacology- Study of Molluscs
 Conchology- Study of Shells.
 Architeuthis-largest invertebrate-commonly called as giant squid
 Inhabited all habitats except Air
 Molluscans came into existence in the early cambrian period and
flourished.

5. GENERAL CHARACTERS:-
 Terrestrial or aquatic( fresh water or Marine water)
 Triploblastic , bilaterally symmetrical, schizocoelomic,
protostomic, unsegmented animals
 Body divisible into head, mantle, foot, and visceral mass.
 Shell when present , usually either external or internal or
absent.
 Coelom is reduced and is represented by pericardial cavity,
gonadal cavity, and around kidney.

6.  Digestive system is complete with digestive gland or
liver(hepatopancreas) Radula – a rasping organ ,
usually present.
 Circulatory system is mainly of closed type, heart is
whith two auricles and one ventricle , blood is with
amoebocytes and haemocyanin
 Respiration is direct or by gills or by lungs or both.
 Excretion is by Paired Metanephridia(Kidneys).

9. Molluscans General characters continued:
Nervous system is of paired ganglia, connectives abd
nerves. Ganglia usually form a circum centric ring.
Sense organs include eyes , statocysts(Balancing
organs) and chemo receptors for touch , smell and taste.
Diecious/ Monoecius: One or two gonads with
gonoducts, opening into renal ducts or to exterior.
Fertilisation either external or internal: development
direct or through free larval forms