116 H. JAMES-CLARK ON THE ANATOMY 



twofold reason : in the first place, because it possesses such an unlooked-for degree of 

 complication in its organization ; and secondly, because it seems to stand intermediate 

 between the two great groups of Ciliata, — the dexiotropic on the one hand, and the Itco- 

 tropic on the other. The transitional forms in all departments of the animal kingdom are 

 eminently suggestive, but none more so than the genus Trichodina. Combining in one 

 animal the typical forms of two groups, and yet so singularly individualistic as to be con- 

 founded neither with the one nor with the other, the elaborate solution of the relations of 

 the various members of its organization to each other, and the tracing of their homologies 

 with those of the groups on either side, engage the attention no less deeply, and none the 

 less worthily, than if it were occupied in the investigation of the most profound philo- 

 sophical problem. 



An attempt, therefore, at a full life-history of this animal becomes an effort at some- 

 thing more than a mere specific description without an aim ; and whatever apparent 

 triviality of detail there may seem to be in it, the consciousness that no one part of an 

 organization is without relation to some other part leads the author to the opinion that 

 an investigator should never undertake to assume what is of importance and what is not. 

 It is no infrequent occurrence that what, at one time, has been deemed worthy of very 

 slight consideration, becomes, at another, the paramount object in a course of scientific 

 research. Nature is not to be represented in full detail by the broad touches and coun- 

 terfeiting portraiture of a Vandyck, howsoever striking and suggestive the likeness may 

 be ; in order to bear a closer inspection, her image must needs be mapped and copied by 

 the more matter-of-fact hand of the humbler Netherlandish artist. 



§ 1. Habitat. 



This species (PI. IV., Trichodina pediculus Ehr.) is found in great abundance creeping over 

 the body, and even to the tips of the tentacles, of our common brown and green fresh- 

 water Hydras (H.fusca and H. riridis Trembley). Oftentimes it may be seen with the 

 middle of its base applied directly over the centre of a group of nettling organs, the 

 former fitting the latter like a cap, and without seeming to disturb the Hydra in the 

 least. 



Notwithstanding the apparent rigidity of the chitinous, uncinate ring of the base, the 

 latter possesses the greatest degree of flexibility, and an unlimited adaptability to what- 

 ever surface it may come upon, no matter how uneven it happens to be. The intimate 

 structure of the chitinous ring does not interfere in the least with, but on the contrary 

 appears to assist in, the flexures of the base. The latter is always the point of attachment ; 

 and upon this part of the body the animal may be seen, almost at all times, gliding to and 

 fro like a miniature cup (figs. 1, 2), now on the upper side of a Hydra, and then on the 

 lower side. At one moment several individuals are crowded together on the tentacles, and 

 in the next instant scattered along its length from base to tip, and giving to it a singularly 

 irregular, changeable outline. At times the Hydra seems to be strangely knotted, and 

 ungainly in outline ; when, upon close examination, we ascertain that it is crowded 

 with a swarm of Keronas, upon several of whose convex backs, one, two, or three Tricho- 

 dinas are seated, enjoying the pleasure of locomotion, without the effort to produce it. 

 Not unfrequently an individual may be seen to leave its reptant mode of progress and 

 take to the surrounding element. Then it swims, at times very swiftly, either in a fully 



