86 Observations upon the Habits Structure and |¥February, 
longitudinal folds arise from the exoderm along either side of the 
entire length of the upper portion of the central cavity, and grow 
outward and downward toward the ventral aspect of the body. 
They soon attain a level with the lower edge of the intestine, 
which, as already mentioned, is much narrower than the pharyn- 
geal section, and those portions of the folds which lie along the 
intestine closely embrace it as a sort of tubular outer coating, and 
unite along their edges into a broad median ridge which extends 
from the anus, where it merges into the exodermic tail expansion, 
‘to a point just in front of the stomachic dilatation. This ridge 
forms the ventral extension of the continuous ventro-dorsal 
median fin. 
The anterior portions of the folds now increase in breadth, 
leaving quite a space, the rudiments of the branchium, be- 
tween their edges and the alimentary tract, and gradually unite 
but with a smooth, unridged surface, along the median line for- 
ward toward their anterior extremities. In this union an opening 
is left between the edges of the folds just at the point where they 
leave the anterior edge of the coalesced ventral ridge; this open- 
ing, which forms a means of communication between the cavity 
enclosed by the folds and the exterior, represents the abdominal 
pore, or branchiopore. It will thus be seen that the abdominal 
portion of the animal presents two widely different sections; the 
one, posterior, with firm walls, closely surrounding the intestine 
and stomach, and with a very restricted cavity enclosed between 
the body walls and this portion of the digestive apparatus ; and 
the other, anterior, with thin dilatable walls, which are at some 
distance from the alimentary tract and which enclose quite an a 
extensive cavity or space, which communicates directly with the 
exterior and with the cavity of the posterior section. It may be 
that these two sections are somewhat more complicated in their 
formation than I have indicated above, that while the posterior — 
portion is fashioned as already mentioned, the walls of the ante- _ 
rior cavity are formed by the outgrowth of new folds from the 
innet edges of the old ones; that is, that after the coalescence of a 
the posterior portions of the first folds around the intestine, these 
folds grow no further, but a new set originate from the inner 
anterior borders of the first, and by the extension and subsequent = 
union of these more delicate laminæ, the walls of the anterior _ 
; seit are formed. The peculiar pit-like character of the bran- 
Se Pere Pe ee ln a” ae Ne a 
