390 MOLLUSC A. 



may express a primitive mode of excretion to which a 

 secondary significance has come to be attached, and in 

 what way carbonate of lime shells are associated with pre- 

 ponderant sluggishness of habit. The thickness of the shell 

 seems often to bear some relation to the external and internal 

 activities of the mollusc, for it is thin in the active scallop 

 (Pecteri) 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 cuttlefish, 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 most cases, as Lang points out, 

 the loss of the shell is justified by increased power of 

 locomotion, by increased adaptation to peculiar habits of 

 life, and so forth. 



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 have particularly described, 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. 



The hard shells of extinct Molluscs are naturally well 

 preserved in the rocks, and long series of fossil forms have 

 been traced with much success. 



