vii MOLLUSCATHE FOOT AND ITS GLANDS 115 



retained. The byssal apparatus may be found, in closely-related forms, sometimes 

 with and sometimes without the byssus itself. In the latter case the foot is 

 generally more strongly developed, and serves for locomotion, i.e. for forcing a way 

 forward into sand or mud, which most of these forms inhabit, or for the saltatory 

 motion of Trigonia. In these cases it is linguiform, or wedge- or hatchet-shaped 

 (Arcidcc, 1 Carditidce, 1 Cyprmidce, 1 Tcllinidce, Scrobiculariidce, Myidce? Cardiidce, 1 

 Lucinidce (foot vermiform), Donacidce, etc.). 



5. When the linguiform, or hatchet-shaped, and often bent, foot becomes more 

 strongly developed as a fleshy and extensible organ, every trace of the byssus and its 

 apparatus disappears, at least in the adult (Unionidce, many Veneridce, Cyrenidce, 

 Psammobiidce, Mesodermatidce, Solenidce, Mactridce). All these live in mud. The fleshy 

 foot of the Solenidce, which is directed forwards, is so strongly developed that it can 

 often no longer be wholly withdrawn into the shell, which therefore gapes anteriorly. 

 The foot is thick and linguiform in Solcnocurtus ; club-shaped and truncated at the 

 tip in Phamts, Cultelhts, Siliqua, and Ensis ; and cylindrical, with an egg-shaped 

 tip, in Solen. 



6. In forms where one of the valves has become firmly attached to some hard 

 substance, the foot (the byssus being absent) may become rudimentary (Chamacea), 

 or may altogether disappear (Ostreidce). In forms which inhabit mud or excavations 

 made by themselves in stone, etc., and which surround the body with an accessory 

 calcareous tube (Gastrochcenidce, Clavagellidce), the foot is also reduced to a small, 

 usually finger-shaped rudiment. The series of boring Pholadidce is specially 

 interesting. Pholas has a pestle- or sucker-shaped foot, which, projecting through 

 the shell cleft, serves to attach the animal while boring. In Pholadidea and 

 Jouannelia only the young while boring their habitations possess such a foot ; 

 as soon as they have finished this work the pedal aperture of the mantle closes, the 

 anterior cleft of the shell is also closed by means of an accessory shell-piece called 

 the callum, and the foot completely atrophies, so that the animals are no longer 

 capable of locomotion. 



In the attached Anomia, also, the foot is small : it is of great importance, how- 

 ever, as bearer of the byssal apparatus. The shelly plug (see p. 63), by means of 

 which the animal is fastened to the ground, and which occupies the deep notch cut 

 by the byssus into the right or under valve, must be regarded as a calcified byssus. 



Many Lamellibranchs (Crenclla, Lima, Modiola) weave a byssus web which they 

 inhabit like a nest, and which they strengthen by the addition of foreign bodies 

 attached by byssus threads. 



E. Cephalopoda. 



The question, what part of the body in Cephalopoda corre- 

 sponds with the foot of other Mollusca, has led to much discussion and 

 careful investigation. It may now be considered as pretty well estab- 

 lished that the foot in Cephalopoda forms : (1) the arms, (2) the siphon. 



The arms are considered as lateral processes of a Molluscan foot 

 which have pushed past the head to the right and left, and have 

 united in front, so that the head is entirely encircled by the foot, and 

 the mouth has come to lie in the middle of the ventral pedal surface, 

 i.e. at the centre of the circle of arms or brachial umbrella. That this 

 circle of arms is a derivative of the foot is supported by important 

 anatomical and ontogenetic facts: (1) The arms are innervated from 



1 Pro parte. 



