242 LIFE BY THE SEASHORE. 



which it attacks by first drilling a round hole in the shell, 

 and then sucking up the soft contents by means of its pro- 

 trusible proboscis. At low tide the dog-periwinkles remain 

 motionless attached to the dry rocks, but they have a curious 

 habit of suddenly relaxing their hold and dropping into 

 the pools beneath. Beneath overhanging rocks their egg 

 capsules may be found at all seasons, sometimes empty 

 and sometimes full, and not infrequently stained with the 

 creature's purple dye. 



The shell is very strong, usually white or pale yellow, 

 with a very large body-whorl and a distinct but short canal, 

 and in the adult reaches a length of over an inch. The 

 surface is usually nearly smooth, but in one variety the 

 lines of increase rise up to form "fringe-like imbricating 

 lamellae," and there are in addition spiral ridges placed very 

 close together. The colour is very variable, the shell being 

 sometimes banded with dark brown and sometimes entirely 

 dark brown ; the shape is also variable. 



The living animal is pale in colour, usually white. Behind 

 the head lies the gland which secretes the colourless fluid 

 from which the purple dye is obtained by exposure to the 

 air. The egg capsules are little oblong, shortly-stalked cups, 

 and are placed in clusters on stones and shells. 



The next two species belong to the genus Nassa, and are 

 usually more abundant as shells on the shore than in the 

 living condition. The shells are prettily marked, and in 

 the young state are often collected by children in quantities 

 to make necklaces or ornaments. Both species are some- 

 times found living near low-tide mark. 



The larger species, Nassa reticulata, has a thick shell of 

 pale brown colour, which may reach a length of one and 

 a half inches. It is covered by numerous convex ribs, 

 which are crossed by spiral grooves, producing a netted 

 appearance. The aperture is prolonged into a short and 

 broad canal. The animal is yellow, speckled with black, 

 and has the foot prolonged into two filaments, usually 

 carried upright when the creature walks. 



The other species, N. incrassata, is much smaller, has the 

 whorls of the shell rounded, and a dark spot placed at the 

 origin of the canal. The aperture is largely filled up by a 

 projection, or varix. 



