MOLLUSCS 



415 



animal itself are exceptional in this group, those forms with a 

 large shell usually finding this a sufficient protection. Many of 

 the creeping marine species shelter in rock-crevices, and others 

 burrow (see p. 217). Some land-snails (species of Helix) exca- 

 vate holes in limestone rocks, and the carnivorous slug Testacella, 

 which pursues earth-worms underground, escapes many dangers 

 by its mode of life. Many of the soft-bodied sea-slugs possess 



ep. 



Fig. 940. Gastropod Larvae, much enlarged 



A, Trochosphere of Limpet (Patella). B-E, Veligers of a Fore-gilled Snail, a Hind-gilled Snail, a Wing-footed 

 Snail, and a Pulmonate (Onchidlum}. br., Brain; d.g., digestive gland; e., ear (otocyst); ep., epipodium; 

 y^,foot; g., gullet; gl., gland; z'., intestine; i.a., intestinal aperture; ;., mouth; op,, operculum; s., shell; s.m., 

 shell-muscle; st., stomach; t., tentacle; v., velum; v.m., muscles of velum. 



disagreeable properties, which are advertised by warning colours, 

 and some marine gastropods resemble their surroundings so 

 closely as to be inconspicuous (see vol. ii, pp. 306 and 285). 

 These devices largely obviate the necessity for dwellings, while 

 egg-capsules and the like play the part of nurseries. 



The typical shape of a Gastropod shell is spiral, and it is 

 commonly large enough to serve as an efficient refuge, the animal 

 completely withdrawing into it, and often possessing a horny or 

 limy plate (operculum) which serves as a door to close the weak 



