VII • MOLLUSCA— MUSCULATURE 119 



(byssus and sole glands). On the other hand, the water was sup- 

 posed to enter the foot through intercellular ducts between the 

 epithelial cells, but this theory has also been disproved. 



Others, again, maintained that the water was conducted by the 

 nephridia to the pericardium, and conveyed thence through the blood 

 vascular system ; but the pericardium has been shown to be entirely 

 separated from the vascular system. Indeed, many theories on this 

 subject have been put forward and disproved. 



It is now the received opinion that, except in the case of one 

 animal, which will be presently described, the foot is swelled by a 

 rush of blood which, flowing into the foot, is prevented from returning 

 to the body by sphincter muscles. 



The exceptional case is that of Natica Josepliina. In this animal 

 there can be no doubt that water is taken in to swell the foot. The 

 swelling takes place very quickly — in less than five minutes. When 

 the foot is stimulated it gives out an amount of water which would 

 fill the empty Natica shell two or three times. The water is taken in 

 through very small slits, invisible to the naked eye (probably indeed 

 through a single very narrow slit, lying at the edge of the foot), and 

 finds its way to a system of water sinuses, quite distinct from all 

 other cavities of the foot, and also distinct from the blood vascular 

 system (which in Natica is closed). There can thus be no question of 

 a direct taking in of water into the circulatory system. The water 

 slits at the edge of the foot can be closed by muscles, which extend 

 from their upper to their lower edges. 



XII. Musculature and Endoskeleton. 



This chapter has for its subject simply the general musculature of the body. It 

 would be impossible to describe in detail the musculature of special organs, such as 

 the intestine, the heart, and the copulatory organs, that of the cutis, or even that of 

 the most muscular of all the organs — the foot ; since, owing to the varied development 

 and functions of this organ, its musculature is liable to innumerable modifications. 



The character of the general body musculature of the Mollusca is 

 determined by the degree of development of the shell, whose function 

 is to protect the soft portions of the body. In order to make this 

 protection complete, the Molluscan body is, as a rule, though differing 

 greatly in details, so arranged that the soft parts can be entirely con- 

 cealed in the shell, which can itself in many cases be closed. The shell 

 thus functions as skeleton and passive locomotory organ, to which are 

 attached such muscles as draw the body into the shell by their contrac- 

 tion, and such as partially or wholly close the shell. 



It is obvious that the arrangement of the musculature becomes 

 much modified secondarily in cases where the shell aborts or altogether 

 disappears. 



The musculature of the Mollusca is not transversely striated. 



