GENERAL NOTES ON MOLLUSCS. 



393 



Best developed in Gasteropods and Cephalopods, the head 

 region may elsewhere be represented, as in DentaUum, 

 merely by a buccal tube fringed with tentacles. Apart from 

 Lamellibranchs, the radula is characteristic and, with few 

 exceptions, universal. 



Almost as important is the condition of the characteristic 

 Molluscan foot. Primitively this had the form of a ventral 

 creeping sole, as shown, for example, in its simplest condition, 

 in Chiton (Fig. 200). This condition is retained in many 

 Gasteropods, and in the simplest Lamellibranchs, like Sole- 

 nomya. In most Lamellibranchs, however, in adaptation 

 to a more or less passive life in the sand, the foot became 



FIG. 194. Common Buckie (Buccinum undatunn. 

 e., Eye ; s. , respiratory siphon ; 0., operculum ;f., foot. 



wedge-shaped, and the characteristic byssus gland, which 

 secretes attaching threads, is developed. In the active 

 cuttles the foot became greatly modified, and in those 

 related to Sepia a portion of it is specialised as the funnel 

 the main organ of active locomotion. That the condition 

 of the foot cannot in itself be employed as a basis of classi- 

 fication, is, however, obvious, when its differences within 

 the limits of a class are considered. Thus it is obsolete in 

 the pelagic Phyllirhoe among Gasteropods, in the sedentary 

 oyster among Lamellibranchs ; in the pelagic Pteropods part 

 of it forms lateral wing-like lobes used in swimming, while 

 in lanthina,) which has a similar habit, its chief use is to 

 secrete a "float" to which the egg-capsules are attached. 



