126 MOLLUSCA. 



skin forming what is known as the mantle ; this generally 

 secretes a calcareous shell, consisting of one or two pieces. 

 There is a ventral protrusion of the body forming the foot, 

 a muscular organ used generally in locomotion. Res- 

 piration takes place in most cases by means of gills ; in 

 some forms there is instead a pulmonary sac. A heart is 

 present and is placed on the dorsal surface, and it is 

 systemic (i.e. receives arterial blood). The mouth is 

 situated anteriorly, and typically the anus is posterior, the 

 former may or may not be provided with organs of 

 mastication. Renal organs (nephridia) are present and 

 communicate with the body-cavity. The nervous system 

 consists of three pairs of ganglia connected by cords. 

 Sexual reproduction only occurs; some forms being uni- 

 sexual, others hermaphrodite. 



The Mollusca are divided into four classes, (1) Lamel- 

 libranchiata, (2) Gasteropoda, (3) Scaphopoda, (4) Cepha- 

 lopoda. 



CLASS. LAMELLIBRANCHIATA. 



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 

 animal, and the two are joined together by means of a 

 hinge and ligament at the dorsal margin. The interior of 

 the shell is lined by a fold of the skin, the mantle (fig. 44 

 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 (6). This is a 

 laterally flattened muscular organ, frequently hatchet 1 or 

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



