106 COMPARATIVE ANATOMY CHAP. 



which were formerly considered to be gills. The base of the lobe here stretches 

 along the whole length of the base of the foot, and is further prolonged posteriorly 

 in the shape of a free appendage with a groove running along it. This process can 

 be protruded beyond the shell, and probably assists in conducting food to the 

 mouth. 



X. The Foot and the Pedal Glands. 



The ventral side of the body in the Mollusca is characterised by the 

 pronounced development of its musculature, which enables the animal 

 to creep, a fleshy foot, provided with a flat sole suited for creeping, 

 distinct from the rest of the body and especially from the head, being 

 developed. This strong ventral musculature must be considered as the 

 remains of the dermo-muscular tube of the racial form, which attained 

 greater development on the ventral side in adaptation to a creeping 

 manner of life, while it degenerated on the dorsal side, being rendered 

 functionless and useless by the hard shell. 



The flat form of the foot with a sole for creeping must be con- 

 sidered the primitive form. Such a foot is found in the Chitonida.' 

 among the Amphineura, in most Gastropoda, and in certain Lamelli- 

 branchia, especially in the Protobranchia, which for other reasons also 

 must be considered the most primitive form of Lamellibrancliia. 



The musculature of the foot and of all parts which become differ- 

 entiated from it are innervated from the pedal ganglia or pedal nerve 

 cords. 



The foot may become much modified in adaptation to various 

 methods of life and of locomotion, in fact, it may entirely lose all 

 resemblance to the primitive organ. It may, by constriction or by 

 the formation of lobes or folds, fall into several parts, of which the 

 following are the most important : 



1. Proceeding from before backward we have the propodium, an 

 anterior portion distinct from the rest, and the metapodium behind 

 the former and seldom very distinct, which carries the operculum 

 when this is present. 



2. From below upward there are the parapodia, lobe-like exten- 

 sions of the edge of the ventral sole, and the epipodium, a projecting 

 ridge or fold round the base, i.e. round the upper portion of the foot. 

 Tentacular processes are often developed on this ridge. 



Taking the different groups in order, the following variations of 

 the foot and the pedal glands (mucous glands and byssus gland) are to 

 be noted. 



A. Amphineura. 



(Of. Section II., p. 29). The foot is here not divided into separate consecutive 

 portions, and there are no parapodia or epipodia. 



