42 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 organisation." In the next stage, the 

 development of an organ, fringed with stronger cilia, and serv- 

 ing both for locomotion and respiration, shows 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 em- 

 bryo is able to move by its own muscular contractions, and to 

 swallow food; it is therefore "hatched," or escapes from the 



Very little is known respecting the development of Brachio- 

 pods. F. Miiller has described * an embryo which, it is thought, 

 may belong to Crania. It possessed two roundish valves of un- 

 equal size, the dorsal being the larger. At the part where the 

 hinge is placed in the adult was a small oval plate. Five pairs 

 of stiff setse projected from the mantle, and four of them origi- 

 nated from the ventral half. The edge of the mantle in the dorsal 

 valve was beset with numerous finer setse, which curved over 

 upon the outside of the ventral valve. The alimentary canal 

 filled the posterior half of the space between the valves. There 

 were two auditory capsules and two eyes. The anterior half was 

 occupied by four pairs of cylindrical arms, surrounding a round 

 knob, at the summit of which was the mouth. Locomotion 

 was effected by means of the cilia enveloping the arms, which 

 impelled the animal through the water with the mouth fore- 

 most. No circulatory or reproductive organs could be detected. 



The young bivalves are hatched before they leave their parent. 

 {See page-S93) The forms they pass through present distinct 

 differences in several families, so that even in the present state of 

 embryological knowledge, some five or six types of development 

 are known. Even in the same family there may be a great dis- 



approaches the inner surface of the vitelline membrane, in order to receive the 

 influence of the spermatozoa ; it then retires to the centre of the yolk, and undergoes \ 

 a series of spontaneous subdivisions. In M. Loven's account it is said to "burst" and I 

 partially dissolve, whilst the egg remains in the ovary, and before impregnation ; it i 

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

 along with the j'olk, whilst the nncleus of tlic genninal 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 

 Bome unobserved manner, without fulfilling any recognised purpose. 



*^ Arckiv fiir Anatomic und Physiologic, 1860, p. 72 ; see also Annals of Nat. Hist 

 for I860. 



