MICROSCOPIC FAUNA. 171 



health and well fed ; their food consists of small Entomostraca and 

 Annelids. But it would be well in supplying food to the Hydra to 

 take greater care than I did on one occasion. I had procured a very 

 fine specimen of H. inridis, and proceeded to write a short paper 

 to read, if there was an opportunity, at the next meeting of our 

 Society ; but reflecting that the meeting would not take place for 

 almost three weeks, I thought it would be as well to give the 

 Hydra some food, and placed in the water with it two specimens 

 of Cando?ia repens. The next morning I found that the dinner 

 had devoured the diner ; the Candona were lively and active, 

 the Hydra was represented by only a small mass of green sarcode. 



Hydra, and especially H. viridis and H. fusca, are infested on 

 their cuticular surface by a parasite of the order Peritricha^ 

 PI. XH., Fig. I. Numbers of these can be seen moving rapidly over 

 the body and tentacles ; they are called Trichodina pediculus. 

 There are other species of the Trichodina found infesting fish 

 and crustaceans, and the principal difference between the marine 

 and fresh-water species, is that the body is discoidal in the fresh- 

 water species, and turban-shaped in the sea species, (see Plate XXL, 

 Figs. 2 and 3.) T. pediculus : the body is conical, discoidal, or 

 hour-glass shaped, according to the conditions of expansion or 

 contraction, the height when fully expanded almost equals, but 

 scarcely ever exceeds the greatest diameter. It moves by means 

 of ciliary wreaths placed on the posterior and adoral margins. 

 The cilia of the posterior margin being the longest, and are 

 inserted on the inner side, and at the base of a thin transparent 

 annular membrane or velum, into which the body of the posterior 

 margin is produced. The horny ring of the acetabulum, Fig. 3, 

 is supplemented by a wreath of horny denticles consisting of an 

 internal and external series, each numbering about 26 ; the denti- 

 cles of the outer series are the larger, and are short, thick, and 

 claw-like; those of the inner set are attenuate, sharply pointed, 

 almost straight, and extending to the centre of the interior 

 discoidal area, composed in each instance of a more solid radial 

 portion, and a membraneous, weblike lateral extension, endoplast, 

 bandlike, or moniliform, curved ; greatest height and diameter 

 when fully expanded one three hundred and sixtieth of an inch. 



In the memoirs of the Boston Society of Natural History, 



