MOLLUSCS WHICH BREATHE IN WATER 



395 



cavity to such an extent that regular entry and exit of water were 



interfered with, while the arrangement which has superseded the 



old one enables fresh water to flow up one side of the gill-cavity, 



over the single gill, and then down the other side, washing 



out the various waste products 



to the exterior. And in the 



carnivorous forms which have 



been taken as examples the 



entering water is provided with 



a special means of access in 



the form of a sort of tube or 



" siphon ". The position of 



this is marked by a notch in 



the shell which, as noticed else- 



where (see p. 96), is charac- 



teristic of carnivorous snails, 



as compared with vegetarian 



forms which have no siphon, 



and therefore no notch to 



lodge it. The difference may 



be plausibly explained if we 



remember that carnivorous 



snails have to be specially 



active in order to get a living, 



and as activity is associated 



with properly -purified blood, 



i 

 We may expect SUCH CreatUreS 



r/-> Viow^ m/~it-^ ^ff^riVo Kt-/^o<4- 



to nave more effective breatn- 

 ing arrangements than their 



o o 



mOre Sluggish relatives Which 



live on vegetable food. 



The breathing organs of the Common Limpet (Patella vul- 

 gata) are in a particularly interesting condition. If we remove 

 the large conical shell which covers the back of this animal, and 

 look for a gill-cavity in the position where it is found in a Whelk 

 or Purple- Shell, we shall readily discover it, though in size it is 

 comparatively small. On opening this cavity, however, no gill 

 is to be found, although some not very distantly-related forms, as, 

 e.g., John Knox's Limpet (Acnuea testudinalis), possess one well- 

 developed gill in this position (fig. 526). Careful examination 



Fig. 525. Diagram or a Whelk (Buccinum), seen from 

 above. Shell removed and the roof of gill-cavity sup- 

 posed transparent i, Mouth; 2, brain-ganglion; 2^, nerve- 



cord connecting sid e.gan g iion (above) with foot-gangiion 

 , (below); 3 ' e of , the t j re " ganglia on the twis f ted newe ' 



loop; 4, gill; 4^, osphradium ; 5, opening of intestine; 

 6 > heart in Pericardium; 8, a gland (purple-gland in Pur- 

 pura); 9, siphon; 10, 10, foot; n, operculutn. 



