416 PHYLUM MOLLUSCA. 
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. 
Larve.—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 /e/ix, partly 
because it is terrestrial; and not in Sega, 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 Chzfon, 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 Hudéma, a semi-parasite. These three classes 
are therefore often placed together as Glossophora or Odontophora, 
in contrast to the Lamellibranchiata (Lipocephala or Acephala), 
