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 penetrated 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, linear channel, PL VII. 

 fig. 4 b, 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 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 lining membrane of the alimentary 

 canal." From the facts just mentioned it would appear that 

 this conjecture is probably correct. 



EXPLANATION OF PLATES VI. and VII. 



Plate VI. 



Figs. 1, 2. Two much-enlarged views of the Hydra from the Northumber- 

 land lakes after development of the sperm-vesicles : a, a, sperm- 

 vesicles ; b, ovum in early stage of development. 



