MODIFICATIONS OP' THE FOOT 



20I 



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

 majority of Pelecypoda, is douhtless derived from a form more 

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

 something of this disc-shaped Ijase actually occurs in some of the 

 Nuculidae, the parapodia heing furnished with pleats wliich 

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

 modifications of the foot are due to its employment as a 

 hurrowing organ. In genera which liurrow l)ut slightly it is 

 small and feebly developed, while in genera which habitually 

 excavate, it l)ecomes the largest and strongest organ of the l)ody. 

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



Fig. 100. — Yoldia Umatula Say, 

 Greenland, sliowing the short 

 plumed branchiae {br, hr), the 

 gasteropodous foot (/), and 

 the large labial palps {l.j}, 

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

 tral margin : B, from the left 

 side, with the mantle turned 

 back ; a.in, f)Osition of an- 

 terior adductor muscle ; i, 

 intestine ; I, liver ; »», in, 

 mantle. 



(p.. 



A B 



ventral to the anterior margin, accompanied l)y a corresponding 

 narrowing of the shell, until it arrives at tlie position seen in 

 Mollusca of the shape of Mya, Pholas, and Solen. In sedentary 

 or attached genera, e.g. Pccten, Cliama, Ostrca, the foot tends to 

 l)ecome aborted. 



The hi/ssi's f/Iand, secreting a byssus of horny tlireads, is 

 characteristic of many Pelecypoda, and may .be observed hj any 

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

 species which do not possess a l)yssus in the adult stage. The 

 pedal gland of many Prosobranchiates, wliicli secretes a tough and 

 almost thread-like slime, is possibly homologous with the I>yssus 

 gland of bivalves. 



Nervous System 



The Mollusca possess a nerv;jus system, which usually consists 

 of a number of nerve centres or ganglia, linked togetlier l)y l)ands 

 (the commissures) and sending out tlu'cad-like nerves whicli 

 ramifv into the various organs. The character of tlie nervous 



