234 ZOOLOGY 



by Fig. 172 B, the stomach is divided into an anterior and 

 posterior (^yloric) portion. The liver forms on each side of 

 the stomach an oval fold, and Communicates 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 (n) 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 idvances, the velum is with- 

 drawn from the under side to the anterior end of the shell. 

 In this condition the Yeliger 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 Loven 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 iTS^Youn 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 

 GlocMdium parasiticum. They are found in the parent 

 mussel during July and August. 



The ship-worm (Teredo navalis Linn. Fig. 174) after the 



