4i6 PHYLUM MOLLUSC A. 



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. 

 Pearls are formed in sacs of the external epithelium of the mantle, 

 sometimes around a centre of a periostracum-like substance, sometimes 

 around the larva of a Trematode or Cestode. They are to be dis- 

 tinguished from concretions formed around an intruded irritant particle. 

 The latter do not show the characteristic lamination of pearls. Some 

 pearl-like structures are fixed to the shell; true pearls are free. While 

 some investigators insist on the parasitic origin of pearls, others are 

 equally emphatic in declaring that they may arise independently. But 

 all are agreed that they are pathological products. 



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. 206). 



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 MOLLUSC A 



Leaving aside the difficult Solenogastres, which may not be Molluscs 

 at all, we may ranlc 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. 

 From primitive forms, related perhaps to Chiton, Mollusca have 

 diverged in two directions. In Gasteropoda, Scaphopoda, and 

 Cephalopods, the head region becomes well developed, and the radula 

 present in the primitive Isopleura is retained, except in rare cases, such 

 as one of the species of Eulima, a semi-parasite. These three classes 

 are therefore often placed together as Glossophora or Odontophora, 

 in contrast to the Lamellibranchiata (Lipocephala or Acephala), 



