VII 



MODIFICATIONS OF THE FOOT 



201 



The pointed axe-shaped foot, which is characteristic of the 

 majority of Pelecypoda, is doubtless derived from a form more 

 akin to the flattened ' sole ' of the Gasteropoda. A foot with 

 something of this disc-shaped base actually occurs in some of 

 the Nuculidae, the parapodia being furnished with pleats which 

 recall similar formations in other Orders (Fig. 100). The 

 principal modifications of the foot are due to its employment as 

 a burrowing organ. In genera which burrow but slightly it is 

 small and feebly developed, while in genera which habitually 

 excavate, it becomes the largest and strongest organ of the 

 body. At the same time it has a tendency to shift its position 



Fig. 100. — Yoldia limatula Say, 

 Greenland, showing the short 

 plumed branchiae {hr, hr) , the 

 gasteropodous foot (/), and 

 the large labial palps (l.p, 

 l.p) : A, as seen from the ven- 

 tral margin ; B, from the left 

 side, with the mantle turned 

 back; a.m, position of an- 

 terior adductor muscle; i, 

 intestine; /, liver; m, m, 

 mantle. 



my~f "■" "" 



A B 



from the ventral to the anterior margin, accompanied by a cor- 

 responding narrowing of the shell, until it arrives at the posi- 

 tion seen in Mollusca of the shape of Mf/a, Pliolas, and Solen. 

 In sedentary or attached genera, e.g. Pecten, Chama, Ostrea^ the 

 foot tends to become aborted. 



The bi/ssus glands secreting a byssus of horny threads, is 

 characteristic of many Pelecypoda, and may be observed by any 

 one in the common mussel. It occurs in the larvae of many 

 species which do not possess a byssus in the adult stage. The 

 pedal gland of many Prosobranchiates, which secretes a tough 

 and almost thread-like slime, is possibly homologous with the 

 byssus gland of bivalves. 



Nervous System 



The Mollusca possess a nervous system, which usually con- 

 sists of a number of nerve centres or ganglia^ linked together by 

 bands (the commissures) and sending out thread-like nerves 

 which ramify into the various organs. The character of the 



