CLASSIFICATION OF MOLLUSC A. 397 



Thus carbonate of ammonia, which seems to occur abundantly in the 

 mantle of perfectly fresh mussels, would, with calcium sulphate, yield 

 carbonate of lime and ammonium sulphate. One cannot suppose that 

 shell-making is expressible in a chemical reaction of this simplicity, but 

 it is certain that Molluscs do not simply absorb carbonate of lime from 

 the sea water, and sweat it out from their skins. It is reasonable to 

 inquire how far shell-making 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 preponderant sluggish- 

 ness 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 [Pecfen] and Lima, thick in the passive oyster 

 and Tridacna, slight or absent in the pelagic Pteropods (" sea-butter- 

 flies ''), 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. 



Larvae. 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." 



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 (Fig. 182). 



But although trochosphere and veliger occur in the 

 development 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. 



CLASSIFICATION OF MOLLUSCA. 



Leaving aside the difficult Solenogastres, which may not be Molluscs 

 at all, we may rank as lowest the Isopleura, bilaterally symmetrical 

 Gasteropods with many primitive characters. Some of these forms, like 

 Chiton^ are probably not far removed from the primitive Mollusca. 



