STRUCTURE AND PHYSIOLOGY OF THE MOLLUSCA. 45 
similarity, as in the case of the marine and fresh-water forms 
of the mytilide. The following account refers to the type to 
which the young of Crenella belong. At first they have a 
swimming disk, fringed with long cilia, and armed with a slen- 
der tentacular filament (flagellum). Ata later period this disk 
disappears progressively as the labial palpi are 
developed; and they acquire a foot, and with it 
the power of spinning a byssus. They now have 
a pair of eyes situated near the labial tentacles 
(Fig. 380* ¢), which are lost at a further stage, or 
replaced by numerous rudimentary organs placed 
more fayourably for vision, on the border of the 
mantle. The development of the young has 
been noticed in many of the genera of Pteropods. 
They are divisible into two groups: those in Fig. 30.* 
which the body is surrounded with one or more rings of cilia, 
and those in which these rings are absent. 
Fig. 30*. Fry of the Mussel.+ 
Most of the aquatic gasteropoda are very minute when 
! Fig. 30. Very young fry of crenella marmorata, Forbes, high'y magnified ; 
d, disk, bordered with cilia; J; flagellum ; vv, valves; mm, ciliated mantle. 
7 Fig. 80*. Fry of mytilus edulis, after Lovén. e, eye; e’, auditory capsule ; Z ¢, 
