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 sery- 
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 
ese. 
Very little is known respecting the development of Brachio- 
pods. F. Muller 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 setee 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 setze, which curyed over 
upon the outside of the ventral yalve. 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 397) 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. Lovén’s account it is said to ‘‘ burst”? and 
partially 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 fulfilling any recognised purpose. 
* Archiv fiir Anatomie und Physiologic, 1860, p. 72; see also Annals of Nat. Hist 
fez 1860. 
