646 ORDER PERITRICHA. 



Trichodina pediculus, Ehr. 

 Pl. XXXI. Figs. 48-52; Pl. XXXIII. Figs. 14-18; and Woodcut, p. 647. 



Body conical, discoidal, or hourglass-shaped, according to the condition 

 of contraction or expansion, the elevation, when fully expanded, equal to, 

 but scarcely exceeding, the greatest diameter ; cilia of the posterior margin 

 exceeding in length those of the adoral wreath, inserted on the inner 

 side and at the base of a thin, transparent, annular membrane or velum, 

 into which the border of the posterior margin is produced ; horny ring 

 of acetabulum supplemented by a wreath of horny denticles consisting of 

 an external and internal series, each numbering about twenty-six, the 

 denticles of the outer series largest, short, thick and claw-like, those of 

 the inner set 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 membranous web-like lateral extension ; 

 endoplast band-like or moniliform, curved. Greatest height and diameter 

 of the extended body 1-360". 



Hab. — Fresh water, infesting the cuticular surface of the fresh-water 

 polypes Hydra fiisca and H. viridis ; sometimes on the branchial appen- 

 dages of the amphibian Triton cristatus. 



This species, which may be regarded as the typical representative of its family 

 and genus, is often found in countless numbers on the surface of the body and 

 tentacles of the fresh-water polypes Hydra viridis and other allied species, to which 

 chosen hosts it either adheres firmly by its posterior acetabulum, or scrambles 

 rapidly over them with the aid of the girdle of cilia developed in the same region ; 

 leaving its adopted fulcrum of support, it is further capable of swimming through the 

 water with considerable velocity. Under these last conditions it may retain its fully 

 expanded form or contract itself into an almost wheel-like shape, and progresses 

 rotating on its axis, or, turning end over end, performs as it were a series of somer- 

 saults. The numerous forms this animalcule is able to assume by reason of the great 

 plasticity of the whole anterior portion of the body, has given rise to much diversity in 

 the accounts and figures of this species handed down to us by the earlier writers. By 

 far the most characteristic attitude presented is that subcylindrical or discoidal one, 

 with a slighdy constricted centre and perfectly parallel anterior and posterior edges, 

 as shown at PL XXXI. Fig. 49, and in which condition, with its somewhat hollowed 

 extremities, it presents a contour that may be suitably compared with that of a dice- 

 box or with a fish's amphicoelous vertebra. Not unfrequently the area within the 

 anterior ciliary wreath or peristome is protruded in a conical form and in such a 

 manner that the entire body presents the hemispherical or turban-shape shown at 

 Pl. XXXIII. Figs. 14 and 15. Stein was the first to show that in addition to the 

 apparently continuous circlet of corneous uncini which forms so conspicuous a feature 

 of the acetabuliform posterior extremity, there exists in the same region, and lying 

 witliout the last-named structure, a second finely striated, flexible, horny, annular 

 ring of a pale amber colour, and which, no doubt, adds materially to the efficacy of 

 the adhesive organ. This second corneous ring, though more transparent than 

 the circlet of uncini, was pronounced by Stein to be of much firmer consistence, 

 since it resisted the action of acetic acid after the latter had become dis- 

 integrated. 



An exhaustive and masterly essay on the anatomy and physiology of Trichodina 

 pediadiis, to which the author is indebted for many points embodied in the foregoing 

 diagnosis, was contributed by Professor H, James-Clark to the ' Memoirs ' of the 



