

MOLLUSCS 143 



sea-weed, where they are easily seen and studied. On the other 

 hand, there is an interesting form which lives higher up on the 

 shore, which does riot lay eggs, but gives birth to shelled young. 

 Why ? Because it lives higher up on the beach than the bladder- 

 wrack zone on which its cousin lays its eggs, and if it were to 

 lay its eggs on the rock the risk of their drying up under the hot 

 sun at low tide would be too great, so it retains them within 

 its body until they hatch. Similar observations can be made 

 with a number of Gasteropod molluscs. It may be said generally 

 that the eggs of these molluscs are easy to hatch in confinement, 

 in which also many species can be readily induced to spawn, and 

 there can be no more interesting exercise than to watch the 

 spawning and hatching processes, and to endeavour to correlate 

 the peculiarities in any particular case with the conditions of 

 life of the species. 



Only a few words can be said about the common shore Gastero- 

 pods themselves. Interesting because of its simple structure 

 and its geological age is Chiton, a very common 

 little form, easily recognised by its eight shell 

 plates. On the under surface one sees the flat 

 creeping foot, and at its side the paired gills. 

 One may also easily see that, much as in a worm, 

 the mouth is near one end of the body, and the 

 anus near the other. Most other Gasteropods, as, FIG. 

 for example, the common limpet, have the anus showing the 

 at the right-hand side of the body near the head, e ! ght she11 



plates. 



that is, show, as compared with worms or with 

 Chiton, a curious twisting of the body. For this and some other 

 reasons naturalists believe that Chiton is a very primitive form, 

 and that, from forms like it the other Gasteropods were evolved. 



As a general rule the common limpet, with its ugly cap- 

 shaped shell, does not thrive in an aquarium, where it seems to 

 miss the ebb and flow T of the tide, but success may sometimes 

 be obtained with its more beautiful allies, as with the delicate 

 Helcion pellucidum, which lives on the fronds of oarweed far out 

 on the rocks, or with the tortoise-shell limpet (Acmcea testudin- 

 alis) , whose mantle is of a pale green colour. 



The periwinkles, dog-whelks and whelks proper, though, 



