234 ZOOLOGY 
by Fig. 172 B, the stomach is divided into an anterior and 
posterior (pyloric) portion. The liver forms on each side of 
the stomach an oval fold, and :ommunicates by a large open- 
ing with its cavity; while the intestine elongates and makes 
more of a bend. ‘The organ of hearing then arises, and be- 
hind it the provisional eyes, each appearing as a vesicle with 
dark pigment corpuscles arranged around a refractive body. 
The nerve-ganglion (x) appears above the stomach. The 
two ciliated gill-lobes now appear, and the number of lobes 
increases gradually to three or four. The foot grows larger, 
and the organ of Bojanus, or kidney, becomes visible. The 
shell now hardens; the mouth advances, the velum is with- 
drawn from the under side to the anterior end of the shell. 
In this condition the Veliger remains for a long time, its long 
flagellum still attached, and used in swimming even after the 
foot has become a creeping organ. Latest of all appears the 
heart, with the blood-vessels. 
Upon throwing off the Veliger condition, the velum con- 
tracts, splits up and Lovén thinks it becomes reduced to the 
two pairs of palpi, which are situated on each side of the 
mouth of the mature Lamellibranch. The provisional eyes 
disappear, and the eyes of the adult arise on the edge of the 
mantle. 
In the fresh-water mussels (Unio) the developmental his- 
tory is more condensed. The velum of the embryo is want- 
ing or exists in a very rudimentary state. The mantle and 
shell are developed very early. The 
young live within the parent fastened to 
each other by their byssus. The shell 
(Fig. 173) differs remarkably from that of 
the adult, being broader than long, trian- 
gular, the apex or outer edge of the shell 
Fig. 173,—Young Unio, hooked, while from different points within 
—After Morse. project a few large, long spines. So dif- 
ferent are these young from the parent that they were sup- 
posed to be parasites, and were described under the name of 
Glochidium parasiticum. They are found in the parent 
mussel during July and August. 
The ship-worm (Teredo navalis Linn. Fig. 174) after the 
