470 PHYLUM MOLLUSCA 



Sphere," 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. 248). 



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. 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 re- 

 tained, 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), where the radula has disappeared, and the head 

 region remains undeveloped. As already seen, however, 

 the lowest Lamellibranchs have a flattened creeping foot 

 and simple feathery gills > in these respects resembling 

 Gasteropods. There is also much reason to believe that 

 the Scaphopoda arose from a stem common to them and 

 the lowest Gasteropods, which are central unspecialised 

 forms. The Cephalopoda are the most highly specialised 

 of all the Mollusca, and in their existing forms at least not 

 nearly related to the other classes. 



