SNAILS AND SLUGS 



319 



round to the front (fig. 182). It is at any rate pretty clear that a 

 spiral hump and shell are more compact and convenient than a 

 much elongated hump covered by an extinguisher-shaped shell. 

 In most cases the twisting, as viewed from above, has taken place 

 in a direction opposed to the hands of a watch, but in some few 

 snails the opposite has been 

 the case, so that the spiral shell 

 is left-handed. 



A little observation at the 

 sea-side will show that Peri- 

 winkles are in the habit of 

 creeping about on the rocks, 

 feeding on sea- weed, from 

 which they are able to scrape 

 small pieces by means of the 

 rasping organ. The part of 

 such an animal which protrudes 

 from the shell will be seen to 

 be bilaterally symmetrical, and 

 to mainly consist of a foot much 

 smaller than that of the Ormer 

 (see p. 307), and a head pro- 

 vided with a prominent snout 

 and two tentacles, each of which 

 bears a small eye at its base in 

 the form of a black spot. The 

 projecting hind-end of the foot 

 bears upon its upper side a 

 horny plate, the operculum, 

 which when the animal is com- 

 pletely withdrawn into the shell 

 by means of the shell -muscle 

 stops up the aperture, thus 

 guarding the only weak point in the defences. The operculum 

 corresponds in shape with the aperture or mouth of the shell, which 

 here, as in vegetarian snails generally, possesses a continuous 

 margin devoid of any notch. 



Examination of the mantle - cavity and the related organs 

 will show several important points of difference from the Ormer 

 (see p. 308). As before, the last part of the intestine can be 



Fig. 182. — Diagram of a Comb-gilled Snail, seen from above. 



Tile roof of mantle-cavity and overlying shell supposed 



transparent. 

 1, Mouth; 2, brain-ganglion; 2^, nerve-cord connecting 

 side-ganglion (above) with foot-ganglion (below) ; 3, one of 

 the three ganglia on the twisted nerve-loop; 4, gill; 4^, os- 

 phradium; s, opening of intestine; 6, heart in pericardium; 

 8, a gland (purple-gland in Purpura); 9, siphon; 10, 10, foot; 

 II, operculum. 



