53 MANUAL OF THE MOLLUSC A. 



Up to tliis point nearly the same appearances are presented!, 

 by the eggs of all classes of animals, — they manifest, so far, a | 

 complete "unity of organization." In the next stage, the de- ] 

 velopment of an organ, fringed with stronger cilia, and serving ' 

 both for locomotion and respii'ation, shews that the embryo is 

 a molluscous animal; and the changes which follow soon point ' 

 out the particular class to which it belongs. The rudimentaiy i 

 head is early distinguishable, by the black eye-specks ; and the ] 

 heart, by its pulsations. The digestive and other organs are first | 

 " sketched out," then become more distinct, and are seen to be : 

 covered with a transparent shell. By this time the embryo is j 

 able to move by its own muscular contractions, and to swallow i 

 food; is is therefore " hatched," or escapes from the egg. ] 



The embryo tunicary quits the egg in the cloacal cavity of its •'■! 

 parent, and is at this time provided with a swimming instrument, | 

 like the tail of the tadpole, and with processes by which it attaches j 

 itself as soon as it finds a suitable situation. \ 



The young bivalves also are hatched before they leave their i 

 parent, either in the gill cavity or in a special sac 

 attached to the gills (as in ci/clas), or in the in- ' 

 terspaces of the external branchial laminae (as in i 

 unio). At first they have a swimming disk, fringed 1 

 with long cilia, and armed with a slender ten- .] 

 tacular filament (Jlagelliim). At a 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 j 



tiaUy dissolve, wMst tlie egg remains in tlie ovary, and before impregnation ; 

 it then passes to the centre of the yolk, and undergoes the changes described i 

 by Barry, along with the yolk, whilst the nucleus of the germinal vesicle, or ' 

 some body exactly resembling it, is seen occupying a smaU prominence on 1 

 the surface of the vitelline membrane, until the metamorphosis of the yolk j 

 is completed, when it disappears, in some unobsei-ved manner, without ful- ' | 

 filling any recognized purpose. 



* Fig. 30. Very young fry of crenella marmorata, Forbes, highly magni- 

 fied ; d, disk, bordered with cilia ;/, flageUum ; v v, valves ; m, cihated mantle. ' 



