MOLLUSCS WHICH BREATHE IN WATER 397 



conveniently situated, being washed with sea-water whenever the 

 tide is up. 



So far we have considered the Fore-gilled Sea-Snails, in 

 which the gill-cavity has been brought right round to the front, 

 as the result of twisting, and the gill or gills which it usually 

 contains are therefore in front of the heart, which they supply 

 with pure blood. But in the Hind-gilled Sea-Snails the upper 

 part of the body has begun to twist back again (in the direction 

 of the hands of a watch) so that the gill-cavity has shifted from 

 the front to the right side of the body. This is the case, for 

 example, in the curious slug-like Sea- Hare (Aplysia), where the 

 laterally-placed gill-cavity contains a single gill which is now 

 placed rather behind the heart (figs. 527 and 528). This group 

 of Molluscs also includes the Sea - Slugs proper, which have 

 struck out a line of their own, and have lost shell, gill-cavity, 

 and typical gills. Some of them, however, have been enterprising 

 enough to grow new 

 gills, just as the Limpet 

 has done. Of this a good 

 example is the Sea- 

 Lemon (Doris), in which 

 a beautiful circlet of 

 feathery gill-plumes is 

 situated far back on the 

 upper side, surrounding 

 the opening of the intes- 

 tine (fig. 529). Although 

 these have no gill-cavity to shelter them they are well-protected, 

 for they can be drawn back into a groove until no trace of them 

 can be seen from the outside. 



Other Sea-Slugs (as Elysia) are content to breathe with the 

 general surface of the body, and this method appears to purify 

 the blood sufficiently in a large number of thin-skinned animals 

 belonging to diverse groups, especially when they are of small 

 size. This is partly dependent on the fact that the smaller the 

 animal the larger the surface of the body relative to its bulk, 

 besides which none of the organs are very far removed from the 

 surrounding medium, and consequently have but little difficulty 

 in exchanging their waste carbonic acid gas for oxygen. 



Fig. 529. Side view of Sea-Lemon (Doris) 



