l88 BY THE DEEP SEA. 



If these beaks have the slightest tendency to either end of 

 the shell, it will be to the front, where we should expect the 

 creature's head to be, if it had one. This point made clear, 

 by reference to the shell we have just picked up, we can say 

 which is the right and which the left valve. The valves are 

 hinged by a band of a substance that looks much like catgut. 

 It is elastic in character, and is always pulling at both valves, 

 so that the natural tendency of the shell is to gape open. But 

 inside the shell there are, in most bivalves, two much more 

 powerful bands of muscular fibres (the oyster has but one), 

 which, by their tension, can slowly or suddenly bring the 

 edges of both valves closely and tightly together, and hold 

 them so for an indefinite period. You can see the marks 

 where these muscles were attached, one at each end of the 

 valve. Between these two marks (" muscular impressions ") 

 there runs a colourless line marking the area to which the 

 mantle was attached ("pallial impression"), but this line is 

 often interrupted, towards the hinder end of the shell, by a 

 bay or sinus (the " pallial sinus "). 



The mantle is a delicate membrane on each side of the 

 mollusk's body, which has the power of forming the shell, to 

 which it is attached save at the margins. The " pallial sinus" 

 is caused by the syphons which are protruded at that end of 

 the shell. At the other end, as shown in the figure of the 

 Banded Venus, is the "foot." The "syphons" are two deli- 

 cate tubes, and if you were to put a living Venus, or other 

 syphon-bearing mollusk into a glass of clear sea-water, and 

 drop a little finely-divided indigo, or other colouring matter, 

 in the immediate neighbourhood of these syphons, you would 

 observe a stream of the minute colour-particles rushing into 

 one of these tubes, and a stream of clear water issuing from 

 the other. The inflowing stream passes between the leaf-like 

 gills, or respiratory organs (" branchiae "), where it is effectually 

 strained, all solid matter being retained and passed on to the 

 stomach, whilst the filtered water passes out through the 



