OF THE STU1.1GE0N. 
36S 
the mucous and serous surfaces communicate directly with 
each other." This statement is applicable to the whole class 
of mammiferous animals in which the peritoneum is per- 
forated in the same manner, and where, consequently, the 
same communication is established between the serous and 
mucous surfaces, and, consequently, between the serous 
surface of the peritoneum and the external or cutaneous 
surface of the body. 
Exactly in this predicament are the orifices of the ray, 
shark, and sturgeon now mentioned. Though at first 
sight extraordinary that there should be a direct commu- 
nication between the external surface of the body and the 
cavity of the peritoneum, on reflection it appears not more 
extraordinary than the communication between the cuta- 
neous and mucous surfaces with the serous in the Mam- 
malia generally. In the males, indeed, of this class, we 
have not yet demonstrated this communication. But, if 
our researches are directed to the period of foetal life, I 
think it not unreasonable to anticipate such a discovery. 
In the Sturgeon, I have already stated my belief that the 
existence of these orifices is connected with the mode in 
which the animals propagate their species. The chief rea- 
son for this opinion is, that in this animal no opening from 
the ovaries exists into the cloaca, as in others of the finny 
tribes. So far as can be judged in an animal so young, I 
regard the smallest specimen as a male, the soft roe or milt 
being seen on each side of the vertebral column, and urinary 
bladder. The large one is evidently female. In neither, 
however, do I find any communication between these parts 
and the cloaca ; and I am therefore inclined to retain the 
opinion originally stated, that the peritoneal orifices are 
the outlets for the spawn and ova of the animals. This 
opinion, I admit, requires confirmation before it be recog- 
nised as an established principle. In the mean time, it 
