GENUS VORTICELLA. 669 



other hand, tending to demonstrate that such markings are mere superficial annula- 

 tions of the cuticle. The entire absence of these transverse markings in a very 

 large proportion of the numerous species, sufficiently indicates their non-essential 

 nature, while further evidence of their correct relegation to the category of surface 

 ornamentation is afforded by the species here described under the title of Vorticella 

 monilata, in which the more usual linear markings are replaced by a closely set 

 series of bead-like elevations. 



The phenomena pertaining to the various processes of reproduction, as mani- 

 fested by the Vorticellida generally, and typically by the representative genus 

 Vorticella, are of high interest, and may, at this point, be appropriately narrated. 

 As among all members of the infusorial tribe, that mode of increase which is 

 most prominently and commonly exhibited takes the form of simple fission, one 

 primary zooid or animalcule giving rise by more or less frequently repeated sub- 

 division to an innumerable progeny of daughter zooids. This fissive process in 

 the genus Vorticella, and, so far as known, with the exception only of the genera 

 Spirochona, Lagenophrys and Ophrydium, throughout the Vorticellidas, takes a longi- 

 tudinal direction. The circumstances attending this mode of increase are nevertheless 

 considerably more complex than those which obtain among the various infusorial groups 

 previously described, and are well exemplified in connection with the delineations of 

 this process in Vorticella marina as given by Greeff and reproduced at PI. XXXV. 

 Figs. 1-5. The initial step towards the consummation of this duplicative act is 

 accomplished through the withdrawal by the animalcule of its ciliary disc and assump- 

 tion of a subspheroidal or pyriform contour, as under the ordinary conditions of 

 contraction. The body itself has by this time become abnormally dilated, and 

 presently develops an indentation or notch in the centre of its anterior border. The 

 contracted and lunate vestibular cleft divides into two equal halves, or as observed 

 by the author in the case of Vorticella ncbulifera such observation being also in 

 accord with that of Everts becomes entirely obliterated, a new vestibular cleft and 

 oral system being developed on each side of the median line. A line of division is 

 now gradually produced from the pre-existing anterior median notch, through the 

 centre of the animalcule's body, cleaving in twain in its onward passage both the 

 spheroidal contractile vesicle and the band-like endoplast. The ultimate result of 

 this cleavage process is the production upon the single stalk of two zooids similar 

 to the original or parent unit, and differing but little from it in point of calibre. 

 While one zooid remains attached to the parent pedicle, and expanding its newly 

 developed peristome pursues its ordinary food-ingesting avocations, its duplicate 

 undergoes a further and highly characteristic metamorphosis. The peristome 

 border more usually remains closely contracted, while a new element is introduced 

 through the development round the posterior region of the body of a girdle of 

 powerful vibratile cilia. These, by their vigorous and continued motion, produce 

 an abnormal tension on the thinnest and weakest point, namely at the junction 

 of the body with the stalk ; this finally giving way, the animalcule is launched into 

 the surrounding water, and propels itself whither it pleases with the assistance of 

 its posteriorly developed ciliary girdle. This nomadic phase of existence is, how- 

 ever, of but short duration ; the natatory motions of the zooid grow more languid, 

 and it soon resorts to creeping over adjacent objects with its posterior border and 

 ciliary circlet applied to their surface, presenting under such auspices, as shown 

 at PI. XXXIV. Fig. 1 8, a considerable resemblance to an ordinary repent Tricho- 

 dina. Having at length lit upon a spot adapted to its tastes or exigencies, it adheres 

 to it by its posterior extremity, and the girdle of temporarily developed locomotive 

 cilia becomes absorbed and entirely obliterated. The peristome-border with its 

 enclosed ciliary disc is now everted, and the stalk rapidly developing, the animalcule 

 is soon in no way distinguishable from the normal sedentary zooid that gave it 

 origin. 



The temporary locomotive phase of Vorticella just described, is, however, by 

 no means exhibited only by zooids liberated through the process of longitudinal 

 fission. The ordinary singly attached animalcules, if submitted to uncongenial 

 conditions, develop in a like manner a supplementary girdle of cilia, and becoming 



