MOLLUSC A. LAMELLTBRANCHIA 197 



of the body is the foot — a muscular organ used in locomo- 

 tion. In most cases respiration takes place by means of 

 gills, which are placed in the cavity enclosed by the 

 mantle. A heart is present, and is placed on the dorsal 

 surface ; it consists usually of a ventricle and two auricles. 

 The mouth is situated anteriorly, and, except in the 

 lamellibranchs, is provided with a rasping organ, the 

 odontophore; the anus, in typical forms, is placed posteriorly. 

 Renal organs (nephridia) are present and place part of 

 the body-cavity in communication with the exterior. The 

 nervous system consists of a ring round the oesophagus, 

 and usually of three main groups of ganglia, from which 

 nerves are given off. Only sexual reproduction occurs ; 

 most forms are unisexual, a few hermaphrodite. 



The Mollusca are divided into four classes : — (1) 

 Lamellibranchia, (2) Gasteropoda, (3) Scaphopoda, (4) 

 Cephalopoda. 



CLASS I. LAMELLIBRANCHIA 



In the lamellibranch, as in the brachiopod, the shell is 

 generally calcareous and consists of two valves, but these 

 instead of being dorsal and ventral as in the latter, are 

 placed one on the right, the other on the left side of the 

 body, and the two are joined together by means of a hinge 

 and a ligament at the dorsal margin. The interior of 

 the shell is lined by a fold of the skin, the mantle (fig 91, 

 m), which is divided into two lobes, one being placed in 

 each valve. In the middle of the space enclosed by the 

 mantle (the mantle-cavity) is the foot (/). This is a 

 laterally flattened muscular organ, frequently hatchet- 1 



1 Hence the name Pelecypoda used by some authors for this class. 



