SIXTH RACE OR PHYLUM OF THE ANIMAL KINGDOM 



MOLLUSCA. 



THE Mollusca are essentially bilaterally symmetrical animals with 

 unsegmented bodies. The ventral wall is thick and muscular, and 

 forms a foot which is used for locomotion, and assumes the most 

 varied shapes. A fold of the body wall forms a circular mantle, which 

 hangs down round the body, enclosing a space which is called the 

 mantle or pallial cavity. This cavity is originally deepest and 

 most spacious posteriorly, and contains, at the sides of the median 

 anus, symmetrically grouped, the two gills and the renal and 

 genital apertures. The dorsal portion of the animal is generally 

 developed into a visceral dome or sac, and is protected down 

 to the edge of the mantle by a shell. The mouth lies at the 

 anterior end of the body and leads into a pharynx, which is usually 

 provided with jaws and a rasp -like organ called the radula. The 

 mesenteron or mid-gut is supplied with a large digestive gland (liver). 

 The secondary coelom (enclosed by its own walls) is reduced, but 

 always persists as a pericardium. The blood vascular system is open, 

 and generally to a great extent lacunar. The heart is dorsal and 

 arterial, and was primitively provided with two symmetrical auricles. 

 The nephridia were originally paired, and in open communication 

 with the pericardium. The central nervous system consists of paired 

 cerebral, pleural, pedal, and visceral ganglia. The Mollusca are either 

 sexually separate or hermaphrodite. The gonads are usually single, 

 with paired or unpaired ducts. In the course of development a 

 modified Trochophora arises from the gastrula ; this is the Veliger 

 larva, typical of the Mollusca. 



These general characteristics of the Molluscan body have to be modified for each 



class. In each class there are series of forms which deviate from the typical 



organisation in some one important point, or in several. The shell may disappear, 



and so may the mantle. Either one or both of the gills or ctenidia may be lost, 



VOL. II B 



