]X INTRODUCTION. [CH, 



when the fish again seizes and sucks it, repeating this 

 operation for hours and sometimes for days, until its 

 victim is exhausted. Whether the fish kills its prey by 

 means of any poisonous injection is a question which 

 might be answered by some observant naturalist who 

 not only has an aquarium but makes good use of it. In 

 consequence of this continual and internecine warfare, 

 an excessive increase in the number of mollusks, both 

 on land and in water, is checked. 



Parasites. — Very little appears to be known with re- 

 spect to the animal and vegetable parasites which are 

 nourished by the juices or excretions of li\dng Mollusca. 

 On land, several kinds of slug are infested by a small 

 white mite {Philodromus li?nacuni, Jenyns), which may 

 often be seen in considerable numbers running actively 

 over their bodies, but apparently not doing the slugs any 

 harm, except, perhaps, in slightly lessening or interfering 

 with their secretion of slime, on which these insects pro- 

 bably feed. The Rev. Leonard Jenyns says that the 

 most striking feature in the history of this kind of para- 

 sitic mite is " the circumstance of its not confining its 

 abode to the external surface of the slug, but often re- 

 tiring within the body of that animal — eftecting its en- 

 trance by means of the lateral foramen which leads to 

 the cavity of the lungs. ^^ And he observes, "Indeed 

 I am inclined to think that this ca^dty is its principal 

 residence, whence it only comes forth occasionally to 

 ramble about the sm-face of the body. In one instance 

 I confined in a close box a slug which, to all appearance, 

 was free from parasites. On opening the box a day or 

 two afterwards, I oliserved very many crawling about the 

 slug externally, all of which would seem to have pro- 

 ceeded from the pulmonary cavity. On another occa- 

 sion I observed these insects runnincj in and out of the 



