MOLLUSCS 



413 



spawn of species of Natica (fig. 938), which are originally fixed 

 by one edge, but have been torn from their attachment. They 

 contain a very large number of eggs, imbedded in a hardened 

 secretion, together with innumerable sand -grains, to which the 

 brown colour is due. 



The Violet- Snail (lanthina), which swims freely through the 



Fig. 937. Spawn of Whelk (B^lcc^n^lm undaium\ X 



Fig. 938. Spawn of Natica 



water on its back, constructs a raft of hardened slime with en- 

 closed air-bubbles (fig. 939), to the under side of which the 

 egg-capsules are attached. The slime is derived from a gland 

 imbedded in the foot, the front part of which during the con- 

 struction of the raft is alternately raised above the surface and 

 depressed below it, bringing down an air -bubble each time. 



Fig. 939. Violet-Snail (lanthina) and Raft, a, Snout; b, tentacles; c, gills; d, raft; e, foot;./; eggs. 



This snail drifts about for long distances, attached to its floating 

 nursery. 



Some of the Bonnet- Limpets (species of Crepidula), with a 

 conical shell twisted into a little spiral at the top, appear to 

 remain stationary, holding on to the underlying rock by means of 

 the foot. The egg-capsules are deposited in the space sheltered 

 by the edge of the shell, and possess very thin walls, in relation 

 to their protected situation. In the Worm -Snails (Vermetus} 

 the egg-capsules are deposited within the mouth of the shell, 



