302 MOLLUSCS. 



{Fecien) and Lima, thick in the passive oyster and Tridacna, 

 slight or absent in the pelagic Pteropods (" sea-butterflies "), 

 and in the more or less active cuttlefishes, but heavy in most 

 of the slowly creeping littoral forms. But that this is only 

 one condition of shell-development is evident in many ways, 

 — for instance when we compare land-snails with slugs ; for 

 the latter, though not more active than the former, are 

 practically shell-less. 



In their life-history most Molluscs pass through two larval 

 stages. The first of these is a pear-shaped or barrel-shaped 

 form, with a curved gut, and with a ring of cilia in front of 

 the mouth. It is a trochosphere, such as that occurring 

 in the development of many " worms." So far there is 

 nothing characteristic. 



Soon, however, the trochosphere grows into a yet more 

 efficiently locomotor form — the veliger. Its head bears a 

 ciliated area or " velum," often produced into retractile 

 lobes ; its body already shows the beginning of " foot " and 

 mantle ; on the dorsal surface lies the little embryonic shell- 

 gland. 



But although trochosphere and veliger occur in the develop- 

 ment of most forms, they do not in any of the three types 

 which we shall afterwards describe, — not in Anodonta partly 

 because it is a fresh-water animal, with a peculiarly adhesive 

 larva of its own, not in Helix partly because it is terrestrial, 

 and not in Sepia partly because the eggs are rich in yolk. 



Finally, be it noted that the hard shells of extinct Molluscs 

 are naturally well preserved in the rocks, and that long series 

 of fossil forms have been traced with much success. 



Class Lamellibranchiata — Bivalves. 



(Syn. Acephala, Conchifera, Pelecypoda, Lipocephala, etc.) 

 e.g.. Cockles, Mussels, Clams and Oysters. 



General Characters of Bivalves. 



The great majority live in the sea, but some have become 

 accustomed to fresh water, by slowly spreading from estuaries 

 to rivers, or by the gradual alteration of inland seas. In 

 habit they are very sluggish, and often quite sedentary, 



