8o COMPARATIVE ANATOMY AND PHYSIOLOGY. 



I. Asiphoniata, such as the mussels and the 

 oysters. 



II. In others the mantle is produced into two more 

 or less elongated siphons (Fig. 37, Siphoniata) and 

 these siphons are either not retractile as in the cockle 

 (Cardium), and the immense Tridacna ; or the siphons 

 can be retracted by special muscles (Simipalliata), 

 as in Pholas, Solen, and Mactra. 



B. In the higher division of the Mollusea the 



Fig. 37. Mi/a arenaria, a Siphonate Lamellibranch. 



ex, Excurrent ; in, inourrent siphon ; a, anterior ; a', posterior adductor muscle; 

 gg, branchiae ; /, foot ; t, labial tentacles ; o, mouth ; s, stomach ; d, intestine; 

 p, muscle of the foot. 



cephalic tentacles and eyes are retained, and within 

 the cavity of the pharynx there is developed a special 

 rasping organ or tongue, the presence of which 

 justifies the name Qlossopliora, which is applied to 

 this series. In. a number of these the foot becomes 

 divided into three well-marked regions, but in the 

 lowest group, 



1. Oastropoda, the foot is ordinarily simple, and 

 only constricted into three regions ; it is broad and 

 flattened. In a large number the body undergoes a 

 twisting round its central axis, in consequence of 

 which the two sides of the body come to be unequally 



