56^ ON THE ANATOMICAL PECULI AFvlTIES 
It is remarked by Cuvier, that though the distribution 
of the peritoneum in fishes is analogous to that of the same 
membrane of the Mammalia^ it presents in the family of the 
RaicB a pecuhar circumstance of organization. Instead of 
being a sac, shut on ail sides, as in the mammalia and the 
reptiles, it is perforated in two places, and communicates 
with the lateral surface of the body by two orifices, one on 
each side of the anus, leading directly into the cavity of the 
abdomen. 
This arrangement is not peculiar to the Ray genus. I 
have seen it in the dog-fish ; it is represented by Sir Eve- 
rard Home in the shark and lamprey ; and it was seen by 
Dr Grant in the sword-fish. In the sturgeon it is found in 
remarkable perfection. In the specimen before me these 
orifices are sufficiently large to admit a good-sized quill, and 
lead directly into the cavity of the peritoneum (o o. Fig. 6.). 
Their outer part is formed by a ring of mucous membrane ; 
their inner is peritoneum ; and it is interesting to remark 
the transition between the mucous and serous surfaces, 
which is here distinctly visible. My chief reason for direct- 
ing attention to them at present, is to show that, singular as 
this structure appears, and at variance as it may seem with 
our ideas of the anatomical configuration of the shut sacs, it 
presents no anomaly which may not be reconciled with the 
general characters of these membranes, and no deviation 
from the rank which they hold in the system of organized 
membranes. 
I have remarked, in my Elements of General Anatomy 
(p. 740.), that the peritoneum in the female presents an ex- 
ception to the character of continuity and non-communica- 
tion of the outer with the inner surface of serous mem- 
branes, in being perforated in two points, corresponding to 
the upper orifice of the Fallopian tubes. I have, at the 
same time, observed that this is the only spot at ^vhicli 
