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 interfered with, while the arrangement which has superseded the 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, 

 we may expect such creatures 

 to have more effective breath- 

 ing arrangements than their 

 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 distandy-related forms, as, 

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

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



Fig. 525. — Diagram ot' a Whelk {Biicdnnm), seen from 

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

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

 cord connecting side-ganglion (above) with foot-ganglion 

 (below); 3, one of the three ganglia on the twisted nerve- 

 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; 11, operculum. 



