52 MANUAL OF THE MOLLUSCA. 



Up to this 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 respiration, shews that the embryo is 

 a molluscous animal; and the changes which follow soon point 

 out the particular class to which it belongs. The rudimentary 

 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 

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

 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 

 itself as soon as it finds a suitable situation. 



The young bivalves also are hatched before they leave their 

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

 attached to the gills (as in cyclas), or in the in- 

 terspaces of the external branchial laminae (as in 

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

 with long cilia, and armed with a slender ten- 

 tacular filament (Jlagellum). 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 



tially dissolve, whilst the egg remains in the ovary, and before impregnation ; 

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

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

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

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

 is completed, when it disappears, in some unobserved manner, without ful- 

 filling any recognized purpose. 



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

 fied ; el, disk, bordered with cilia if, fiagellum ; v v 3 valves ; m, ciliated mantle. 



