288 Mr. A. Hancock on a species of Hydra 
issuing from its pointed extremity, belongs to one of them about 
to be exserted. 
We thus see that Hydra is provided with a most efficient 
stinging apparatus, which having penctrated the surface of its 
prey remains fixed there, discharging into the wound its poison- 
bearing filaments. No wonder then that the embrace of these 
animals should be so deadly to the animalcule that comes within - 
their reach; and that the worm so tenacious of life should fall 
paralysed from their touch and die, as we are told, almost without 
a struggle. 
The captor organs of Hydra viridis are exactly similar to those 
just described, but are scarcely more than half their size. In 
this species, too, they are cast from the tentacle. 
Corda considers the arrow-head, and what he calls the ovate 
corpuscle, which we have seen is the membranous stalk sup- 
porting 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. It is therefore evident that 
neither of these parts is calcareous: the arrow-head and barbs 
are probably composed 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 like- 
wise 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, lear channel, P]. VII. 
fig. 4b, 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, ¢, of excrementi-— 
tious matter composed of a tenacious mucus imbedding a granu- 
lar 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 
excretions of the villous linmg membrane of the alimentary 
canal.” From the facts just mentioned it would appear that 
this conjecture is probably correct. 
EXPLANATION OF PLATES VI. anv VII. 
Puate VI. 
Figs. 1, 2. Two much-enlarged views of the Hydra from the Northumber- 
land lakes after development of the sperm-vesicles: @, a, sperm- 
vesicles ; 0, ovum in early stage of development. 
