414 MR. HANCOCK ON A SPECIES OF HYDRA, 
corpuscle, whicn we have seen is the membranous stalk supporting 
the barbs, to be calcareous. Acetic acid, however, has no effect 
on these parts ; and they resist nitric acid for some time, but, in 
the course of an hour or two, almost disappear under the influence 
of this powerful fluid. Itis therefore evident that neither of these 
parts is calcareous: the arrow-head and barbs are probably com- 
posed of horny tissue, or some other substance with which we are 
unacquainted. 
It appears that Corda has also determined the existence of an 
anal outlet at the posterior extremity of the animal. I have 
likewise seen what I take to be a similar outlet. On examining 
a specimen in a highly contracted state, and which was about to 
discharge an egg, a distinct, constricted, linear channel, Pl. VIII. 
fiz. 4 6, was observed passing from the digestive cavity through 
the substance of the adhesive disc, apparently about its centre. 
From this channel issued a long, linear mass c, of excrementitious 
matter composed of a tenacious mucus, imbedding a granular 
substance, resembling both in colour and texture that which lined 
the digestive cavity. 
The true nature of this outlet is enigmatical, since it is known 
that the refuse of digestion is discharged by the oral orifice. 
Professor Owen suggests that “it may give passage to certain 
execrations of the villous lining membrane of the alimentary 
canal.” From the facts just mentioned, it would appear that this 
conjecture is probably correct. 
EXPLANATION OF PLATES VII. anp VIII. 
Puate VII. 
Figs. 1, 2.—Two much enlarged views of the Zydra from the Northumber- 
land lakes after development of the sperm vesicles: aa, sperm 
vesicles; 6, ovum in early stage of development. 
Figs. 8, 4.—Two much enlarged views of the same before development of 
the sperm vesicles. 
Fig. 5.—Two highly magnified views of the terminal portion of the tentacle, 
exhibiting nodular enlargements a, and terminal bulb or 
nodule 0. 
