466 



PHYLUM MOLLUSCA 



region, usually furnished with tentacles and eyes, and con- 

 taining within it a pharynx and radula, is always present. 

 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. 270). This condition is retained 

 in many Gasteropods, and in the simplest Lamellibranchs, 



Fig. 264. — Common buckie {Buccinum undatum). 

 e., Eye ; s., respiratory siphon ; o., operculum ; /., foot. 



like Solenomya. In most Lamellibranchs, however, in 

 adaptation to a more or less passive life in the sand, the 

 foot became wedge-shaped, and the characteristic byssus 

 gland, which secretes attaching threads, was developed. In 

 the Cephalopods 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 Phyllirhoe among Gasteropods, in 

 the sedentary oyster among Lamellibranchs ; in the pelagic 

 Pteropods part of it forms lateral wing-like lobes used in 



