112 COMPARATIVE ANATOMY CHAP. 



Besides these two, other pedal glands are occasionally found. Only one need be 

 mentioned, which is found in some Opisthobranchia (Pleurobranchus, Pleurobranchcea, 

 Pleurophyllidia). It lies at the posterior end of the sole, and consists of glandular 

 caeca, each of which opens separately. 



C. Seaphopoda. 



The foot of Dentalium (Fig. 101) is almost cylindrical; it projects 

 downwards into the tubular mantle cavity, and can be protruded 

 through its lower aperture. The free end of the foot is conical ; the 

 base of the cone carries on each side a fold or ridge which has been 

 compared, with questionable propriety, to an epipodium. These two 

 lateral folds or ridges encircle the base of the conical end without 

 uniting either anteriorly or posteriorly. A groove runs along the 

 anterior middle line of the foot. 



In Siphonodentalium both this groove and the lateral lobes are 

 wanting, and the anterior end of the foot is broadened into a round 

 disc carrying on its edge small conical papillae. 



D. Lamellibranehia. 



The foot in this class is, as a rule, laterally compressed, and has a 

 sharp edge directed downwards and forwards, which can be stretched 

 out beyond the shell. It may be called hatchet-shaped (Pelecypoda) or 

 linguiform, and is especially suited for forcing its way into mud by 

 means of alternate contraction and expansion. 



This peculiar shape must be considered as acquired. Originally 

 the foot of the Lamellibranehia also possessed a flat sole for creeping. 

 The Protobranchia, in fact, have a foot with a ventral disc (Fig. 21, 

 p. 1 4), and so has Pectunculus. The edge of this pedal disc is notched 

 or toothed. When the foot is retracted, this disc folds down the 

 middle line. 



The foot in the Lamellibranehia varies much in details, according 

 to the manner of life or of locomotion of the animal, and according to 

 the development of the byssus. One of the special characteristics of 

 the Lamellibranchiate foot is the gland which secretes the byssus, the 

 latter being a bundle of tough threads varying in thickness, and 

 resembling horn in their physical properties. The Lamellibranch, with 

 these threads, anchors itself to foreign objects. The byssus can 

 generally be thrown off and replaced by a new one, and many forms 

 can move about on a smooth perpendicular pane of glass by means of 

 alternate attachment and rejection of portions of the byssus applied 

 by means of the foot. 



Stationary bivalves, i.e. those attached by one of the shell valves, 

 are in the first instance attached by means of the byssus, for a byssus 

 is, as a rule, present in the young stages of those bivalves which do 

 not possess it as adults. 



