412 PHYLUM MOLLUSCA. 
pharynx with all its associated structures. Elsewhere a head 
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 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 
Fic. 222.—Common buckie (Buccénum undatum), 
e., Eye; s., respiratory siphon ; ¢., operculum ; 7, foot. 
condition, in C/zton (Fig. 228). This condition is retained 
in many Gasteropods, and in the simplest Lamellibranchs, 
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, is developed. In 
the Cephalopods the foot became greatly modified, and in 
those related to Sefza 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 emp‘oyed 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 Thus it is 
obsolete in the pelagic Phy//irhoé among Gasteropods, in 
