go Observations upon the Habits, Structure and [ February, 
the segmentation of the arches, each bar becomes itself divided, 
in time, into two halves, or at least marked, as seen in the adult, 
by a distinct central line of division, Fig. 4, Pl. 1. In all of the 
arches a circlet of long cilia appear around the inner edge as 
soon as the aperture is formed, and, with the growth of the cen- 
tral bars, the circlet gradually extends so as in the adult to nearly 
if not quite curtain each cleft, and when in motion form such 
admirable strainers for these openings, as has been indicated when 
speaking of the habits of these animals. The movement of the 
cilia is wave-like and continuous around. the edge of the slit, with 
the stress or emphasis of the motion always in the same direction. 
In the meantime, while the left branchial arches have been en- 
larging and dividing, the first three or four pharyngeal openings 
have attained a counterbalancing position upon the right side, and 
have, similar to those of the left side, become divided into branchial 
slits by the downward growth of central bars, and the remaining 
pharyngeal openings have entirely disappeared, so that at this 
period both sides are pierced by ten to twelve similar alternating 
clefts, the first one of the right side being formed like the corres- 
ponding one of the left side, after one or two of the old apertures 
have become divided into clefts, although it is possible that in 
some instances this first slit may be the first pharyngeal opening, 
which has remained mall and undivided, merely increasing 
slightly in length in the direction of the breadth of the pharynx, © 
_ with the general increase in length of the other clefts. The sub- 
sequent arches of the right side are of similar formation to those of 
the left, that is, they are new openings, which are formed through 
the wall of the pharynx, and in all probability none of them have 
any relation to the five or six pharyngeal openings which closed 
up during the changes of the first three or four. Accompanying 
these modificatiens which take place in the arches, there is an en- — 
largement of the ventral portion of the center of the alimentary — 
tract, and a forward movement of the stomach, so that this organ 
comes finally to lie within the enlarged anterior cavity of the — 
abdomen, rather than within that portion closely embraced by the 
side walls, . 
The enlargement of the alimentary tract is, at first, in the form 
of a mere swelling-out of one side, accompanied by a pigmen- 
tation of the cells of the walls of this section of the tract, but, in 
a short time a diverticulum, with greenish walls is formed, which — 
