670 ORDER PERITRICHA. 



disconnected from their pedicles, swim off in search of a more suitable location. 

 Similar phenomena are furthermore common to all the compound stock-building 

 representatives of this family group, in which, as familiarly illustrated by such 

 examples as Epistylis digitalis and E. anastatica, the entire branching zooden- 

 drium may become as bare as a leafless tree, through a parallel migration of its 

 associated zooids, within a brief interval of its transfer from its native pond to the 

 stage of the microscope. A peculiar modification of this migratory phenomenon 

 obtains in the genus Zoothamnium, where, in certain species, zooids of abnormal 

 size are periodically produced, which developing in a similar manner a posterior 

 circlet of locomotive cilia, swim off to lay the foundations of new colonies. In 

 intimate connection with the process of equal or subequal longitudinal subdivision 

 already recounted, has to be recorded the production, through a closely parallel 

 course of development, of the comparatively minute bud-like zooids, whose ultimate 

 mission is not that of establishing itself independently and growing to the parent 

 form, but as first demonstrated by Stein, and since confirmed by the more recent 

 investigations of Greeff, Engelmann, and various other authorities of contracting 

 intimate fusion or genetic union with some other normally developed sedentary 

 animalcule. Such a more minute animalcule or microzooid is, as shown at PI. XXXV. 

 Figs. 10-12, usually developed under conditions precisely parallel to those already 

 related of ordinary fission ; not unfrequently, however, these microzooids are pro- 

 duced through a further repetition of the duplicative process, in connection with 

 one of the two equal or subequal primarily separated moieties, the result being, 

 as shown at PI. XXXIV. Fig. 230 and PI. XXXV. Fig. 19, the production of two 

 or it may be as many as eight such microzooids, each with its posteriorly developed 

 ciliary girdle, that remain temporarily attached to the parent stalk. 



In the compound stock-form Epistylis flavicans, according to Greeff, whose illus- 

 trations are reproduced at PI. XXXVIII. Fig. i rr, the entire parent animalcule 

 becomes subdivided into four or eight locomotive microzooids. Having separated 

 themselves from the parent stem, these microzooids swim hither and thither, until 

 they come into contact with a normal sedentary Vorticella^ to whose lateral periphery 

 it then affixes itself by its anterior or oral pole, and gradually boring its way into its 

 interior, the two becoming indissolubly and indistinguishably united. The accom- 

 plishment of this conjugative act is undoubtedly directly comparable with and 

 analogous to the union of the male and female elements, or germ-cell and sperm-cell, 

 of the higher Metazoa. The phenomena succeeding such conjugation are, further- 

 more, physiologically, and indeed morphologically, identical. While it is now made 

 manifest that the sexual theory, as originally propounded by Balbiani, and referred 

 to at length in the preceding volume, see page 94, cannot be maintained, and in 

 which distinct sexual elements were premised to be represented in, and exchanged 

 between, each conjugating zooid, the ovum and spermatozoon are here practically 

 represented in each animalcule, themselves simple cells, and whose union confers, 

 in a like manner, upon the former a renewed capacity of multiplication by binary 

 subdivision. According to the recent investigations of Biitschli and Engelmann, it 

 is only such a rejuvenating influence, or capacity to further subdivide by longitudinal 

 fission, that is accomplished by the conjugative act. It would seem highly probable, 

 however the supposition being further substantially supported by what follows upon 

 the conjugative process among the Flagellata that if not directly, yet more or less 

 remotely, this genetic union of the migrant and sedentary zooids brings about that 

 mode of multiplication by encystment, and the subdivision of the entire body into 

 sporular elements, that yet remains to be described. This phenomenon, while pre- 

 viously recorded by numerous investigators, has been as yet traced with the nearest 

 approach to completeness by Eduard Everts.* As reported by the last-named 

 authority in the case of Vorticella nebulifera, it is an animalcule set free by the 

 process of ordinary longitudinal fission, as already described, that enters upon this 

 reproductive phase of encystment. It, however, takes place quite as, if not more, 



" Untersuchungen an Vorticella nebulifera" 'Zeit. Wiss. Zool.,' Bd. xxxiii., 1870. 



