672 ORDER PERITRICHA. 



of reproduction just recounted represents but a slight modification only of the 

 phenomenon of encystment and endogenous multiplication so prevalent among the 

 class Flagellata, and where, as exemplified more especially in such forms as Heteromita 

 lens, Spirornonas angustata, and Etiglena viridis, such a reproductive process may 

 take place either with or without the intercalation of a conjugative act. The only 

 recognizable distinction between the two correlated processes is manifested by the 

 fact that, while in the Flagellate species the reproductive germs or spores are pro- 

 duced by the segmentation of the complete body-mass pari passu with that of 

 the nucleus, and a portion of which is enclosed within each protoplasmic fragment, 

 after the manner of normal cell-multiplication, in the case of Vorticella the endo- 

 plast alone, increasing at the expense of, and assimilating the body protoplasm into 

 its own substance, undergoes segmentation. The result ultimately arrived at is 

 in both cases precisely identical, the mode of their accomplishment alone being 

 divergent. As it obtains in the group of the Vorticellidae, this reproductive process 

 may be further accepted as yielding a condition intermediate between that simplest 

 form of endogenous multiplication, just cited as encountered among the simpler 

 Flagellata, and that more complicated form which obtains in the Heterotrichous 

 genera Stentor, Spirostomum, and some few Peritricha, such as Didinium. In all of 

 these the band-like endoplast or nucleus assumes the character of an internal 

 proliferous stolon, the germs produced through its disintegration being liberated 

 independently, and in association with the unimpaired maintenance by the parent of 

 its normal form and vital functions. 



In addition to the reproductive form of encystment just recounted, that simpler 

 phase of the same process, assumed merely as a temporary or " resting state " during 

 seasons of drought or other uncongenial conditions, is in common with almost all 

 other infusorial forms, abundantly displayed by the members of the present genus. 

 An illustration of this phenomenon, as exhibited by Vorticella infusionum, is repre- 

 sented in company with the encysted and sporular conditions of various Flagellate 

 types, as attached to hay-fibres, in Fig. i a a of PI. XI. Not unfrequently, as 

 obs'erved by Stein and Everts, these encysted Vorticdla fall a prey to parasitic 

 flagellate organisms, apparently allied to or identical with Heteromita caudata, 

 which having gained access to the body of their host previous to its encystment, 

 subsequently escape from the cyst in place of the original tenant, much after the 

 manner that a brood of Ichneumon-flies swarms out of the pupa case of some 

 Lepidopterous insect. 



A rare form of conjugation, hitherto unnoticed, has been reported by Stein and 

 Claparede and Lachmann, in which two, or according to the last-named autho- 

 rities, it may be even three, sedentary zooids accomplish an intimate union with 

 one another, and, developing a posterior circlet of cilia, become detached from their 

 stalks and finally enter upon an encysted state. Examples of this form of conjuga- 

 tion, as exhibited by the cosmopolitan type Vorticella microstoma, are reproduced 

 from Claparede and Lachmann's work at PI. XXXV. Figs. 22-24. Both here and 

 in the case of the conjugation or zygosis between a migrant and sedentary zooid, 

 the tendency to conjugate, while of rare occurrence, spreads like an epidemic 

 throughout every neighbouring colony, and may therefore, where once detected, be 

 usually examined in detail in a mass of instances. 



On account of the very considerable number of species that have been added 

 as the result of recent investigation, and of the great similarity in external contour 

 which in many cases subsists between them, the genus Vorticella has undoubtedly 

 become one of the most numerically abundant specific groups of the entire infusorial 

 class, while its members present to the working microscopist difficulties in connec- 

 tion with their correct identification closely parallel to those experienced by the 

 botanist in association with the genus Carex. In order to lighten his labours as far 

 as possible in this direction, 'a scheme, supplemented by PI. XLIX., is here sub- 

 mitted, which, while susceptible of considerable extension and improvement, will 

 afford a convenient clue to the elimination of the more formidable obscurities. As 

 a preliminary step towards the accomplishment of the end in view the some three 

 dozen or more species belonging to the^enus are divided into two primary sections, 



