648 ORDER PERITRICHA. 



Busch,* his figure of the same, as here reproduced, PI. XXXI. Fig. 52, being at 

 that time interpreted as an example of gemmation. An approximate estimate of the 

 exceedingly diverse contours that may be presented by Trichodina pedicnlus under 

 varying conditions may be gained through a reference to the numerous figures of the 

 species given in the accompanying illustrations, and among which will also be found 

 a representation of its characteristic aspect as crowded socially upon the portion of a 

 tentacle of Hydra vulgaris. A somewhat remarkable isomorphic likeness is found to 

 subsist between the representatives of the genus Trichodina, as typified by the present 

 species, and the fiee-swimming embryos, or so-called " trochospheres," of certain 

 Polyzoa; a representation of one such larva, reproduced from a drawing placed 

 at the author's disposal by Mr. H. E. Forrest, is delineated for the purpose of com- 

 parison at PI. XXXI. Fig. 53. 



Examples of Trichodina pediculus have been recently obtained by the author from 

 a new habitat, namely, in company with Spirochona tinti?malnilum on the branchial 

 appendages of larvae of Triton cristatus remitted him from the neighbourhood of 

 Dundee by Mr. John Hood in December 1880. Through a careful investigation of 

 the horny armature of the acetabulum, the author has been able to entirely confirm 

 Professor Clark's structural interpretation of this element embodied in Fig. i of the 

 accompanying woodcut, on all points save one, which relates to the development of 

 the membranous expansions or webs of the internal teeth or radii. According to 

 Professor Clark, see Woodcut, Fig. 3 i, these webs are produced on the inner and 

 larger of the two angles formed by the juncture of the shaft or transverse piece, 

 while in those examined by the author, obtained from the triton, such structure, as 

 represented in PI. XXXIII. Fig. 18, is develojied on the external or smaller of these 

 angles. This composition of the structure which appears at first sight, or without 

 the aid of a very high magnifying power, to be a simple continuous denticulate ring 

 out of a number of separate elements, as first demonstrated by the American 

 authority and confirmed by the author's observation, accounts for its rapid dis- 

 integration under treatment with acetic acid, previously recorded. As observed by 

 Professor Clark, the more delicate striations of the posterior face of the second 

 simple ring afford a suitable test object for the resolving qualities of a one-eighth 

 inch objective. The structure, indeed, in its isolated form, bears no inconsiderable 

 resemblance to the discoidal frustules of certain diatoms. Taken in its entirety, the 

 complex adhesive apparatus of Trichodina, with its horny ring and denticles, would 

 seem to find its nearest parallel in the adherent organs or acetabula of the Cephalo- 

 poda, which are likewise commonly strengthened by a correspondingly striated, 

 and for the most part denticulate, horny annulus. The structure as reproduced, or 

 more correctly pretypified, in this minute animalcule is, however, the more remark- 

 able as being the product of a unicellular organism. The Hydra: infested with 

 Trichodina pediculus often entertain as a second infusorial guest numbers of the 

 Hypotrichous form Kerona polyporum. Where the two abound it may be not un- 

 frequently observed that the TrichodincB mount upon the backs of their companions, 

 and thus utilize them, as a man might a horse, for the enjoyment of locomotion 

 without having to participate in the labour of its production. It is of interest to 

 record that so late as the year 1850,! Professor Louis Agassiz made the attempt to 

 demonstrate that the parasitic Trichodina; represented the medusoid form of the 

 a.iioc\3Xtil Hydice, while still later J the same authority proposed to identify the 

 ordinary Vorticellidae as an aberrant group of the Polyzoa. 



Trichodina Steinii, C. & L. Pl. XXXI. Fig. 45. 



This species agrees in all respects with T. pediculus, with the excep- 

 tion of the structure of the posterior horny ring, which, according to 

 Clapar^de and Lachmann, and as represented on the accompanying illus- 



* 'MuUer's Archiv,' 1854. t ' Proceedings Boston Society of Natural History,' p. 354. 



X ' Essay on Classification,' 1857. 



