364 



MOLLUSC A. 



Best developed in Gasteropods and Cephalopods, the head 

 region may elsewhere be represented, as in Dentalium, 

 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. 165). 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 wedge- 



Fig. 158. — Common Buckie (Buccinmn undatvm). 

 c.j Eye ; o., respiratory siphon ; o., operculum ;/., foot. 



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 classification, is, however, 

 obvious, when its differences within the limits of a class are 

 considered. Thus it is obsolete in the pelagic Phyllirho'e 

 among Gasteropods, in the sedentary oyster among Lamelli- 

 branchs ; 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. In various Lamelli- 



