620 ORDER PERITRICHA. 



Order III. PERITRICHA, Stein. 



Animalcules free-swimming or attached, solitary or united in social 

 colonies, often in the latter instance forming branched tree-like growths ; 

 oral aperture terminal or subterminal ; ciliary system consisting of an 

 anterior, circular or spiral, adoral wreath with occasionally one or more 

 supplementary equatorial or postero-terminal locomotive circlets, the 

 remaining cuticular surface entirely smooth ; in those instances in which 

 the adoral wreath takes a spiral form the right limb more usually involute 

 and descending into the oral fossa ; anal aperture posteriorly located or 

 debouching upon the vestibular or oral fossa ; endoplast mostly elongate, 

 band-like; multiplying by longitudinal or transverse fission. Inhabiting 

 salt and fresh water. Trichocysts rare. 



Excepting for the presence of one or two aberrant types, the representatives of 

 the Peritricha constitute collectively the most sharply defined and natural group of 

 the class Ciliata. The smooth rounded bodies and restriction of the cilia to a circular 

 or spiral anterior wreath, and an occasional median or posterior circlet affords 

 a ready clue to their recognition, while the fixed and sedentary life led by the larger 

 proportion of its members affords facilities for their examination met with in no 

 other infusional section. In addition to the very substantial augmentation of the 

 number of generic groups embodied in this treatise, a considerable departure is 

 made in the subdivision of this order into families, as here pursued, from the plan 

 adopted by Stein in the second volume of his ' Organismus/ and reproduced at 

 page 210 of vol. i. Among the more important alterations accomplished in this 

 direction, Tintinnus and its allies have been removed to the preceding order of the 

 Heterotricha, the fine ciliation of their entire cuticular surface, as demonstrated 

 through the researches of Claparede and Lachmann, precluding their further reten- 

 tion in that of the present order. Through the Peritrichous Strombidium and 

 Heterotrichous Strombidinopsis the more remote affinity of Tintinnus to the typical 

 Peritricha is, however, doubtless indicated ; a similar amalgamation of the two 

 orders being likewise apparently brought about between the three genera Codonella, 

 Dictyocysta, and Petalotricha. The family of the Vorticellidae is here made to include 

 the three sections of equivalent value comprised by Stein under the respective titles 

 of the Vorticellina, Ophrydina, and Spirochonina ; all of these, as shown in the 

 succeeding descriptions, passing from one to the other by easy stages of gradation, 

 and preserving at the outside distinctive features sufficient only for their separation 

 into groups holding the position of sub-families. Among the more aberrant types 

 included in the order Peritricha, the three genera Torquatella, Actinobolus, and 

 Urocentrum may be especially alluded to ; the abnormal features of these forms and 

 the question of their more immediate apparent affinities being discussed at length in 

 their generic or specific description. One of these, Urocentrum, corresponding 

 closely in general form and structure with the entirely ciliate Calceolus, may be 

 cited as an additional annectant form between the two orders of the Heterotricha 

 and Peritricha. 



By most authorities the order of the Peritricha has been regarded as typifying 

 the highest and most specialized group of the Infusoria Ciliata. The present 

 author, however, is not disposed to coincide with this view. Specialization appa- 

 rently reaches its zenith of development in the succeeding order of the Hypotricha, 

 in which the cilia or their modified homologues are differentiated and made sub- 

 servient to the performance of a variety of functions to an extent that is not 

 found among any other representatives of the sub-kingdom Protozoa. Evi- 

 dence in support of the lower comparative position occupied by the Peritricha 



