T. W. Engelmaun on the Infusoria. 243 



spherical nuclei. In such instances Engelmann surmises that 

 the act of conjugation has occurred; and he also finds parallel 

 examples in Oxy trichina. The process of conjugation he further 

 noticed partially on one occasion in Vorticella Convallaria, in V. 

 microstoma, and in Urocentrum Turbo. 



Stein has figured Chilodon cucullulus in conjugation, and En- 

 gelmann has witnessed several stages of the process. Among 

 specimens of this animalcule, he met with some having the dor- 

 sum of one individual united with the ventral surface of another, 

 and, again, other specimens with the ventral surfaces in apposi- 

 tion and the oral openings closely conjoined. He confirms the 

 existence of rod-like corpuscles in the nucleus of this species, 

 and remarks on some examples where the nucleus had become 

 spherical and contained such corpuscles scattered irregularly 

 within it, and resembling in figure those of Paramecium aurelia, 

 but only one-half their length. 



Stein observed conjugation in Aspidisca lynceus and A. cos- 

 fata ; to these Engelmann adds A. turrita, and states that he has 

 met with examples where the right border of the ventral aspect 

 of one animalcule has overlain the left border of the dorsum of 

 the other. He questions whether this position represents a later 

 stage of development, since, at the first, the left side of the right 

 animalcule is opposed to the right border of the dorsum of the 

 left animalcule, as is seen likewise inOxytrichina(Pl.III.figs.4-6). 

 The nucleolus of Aspidiscina, previously overlooked by observers, 

 is stated by Engelmann to be of comparatively large size, and al- 

 ways placed, as in Euplotes, more or less close to the middle of the 

 left side of the body, and in intimate contiguity with the horse- 

 shoe-shaped nucleus. This last sometimes presents, at no great 

 distance from each of its two extremities, a transverse fissure-like 

 cavity, such as Stein has figured in Euplotes and Oxytrichina. 



Conjugation was noticed by Ehrenberg in Euplotes, but mis- 

 taken by him for longitudinal fission. Stein and Balbiani have 

 since studied the process in this genus, arriving at the conclusion 

 that it is an act of sexual reproduction. Commonly the two in- 

 dividuals are united along the left of their ventral aspect (fig. 1) ; 

 the nucleus is horseshoe- shaped, the nucleolus distinct from it, 

 and eventually divided into two progressively receding segments. 

 The conjugation is limited to the portion of the body anterior to 

 the peristome. Among such conjugated ordinary specimens are 

 others which present a newly-formed adoral fringe of cilia on the 

 posterior half of the ventral surface, and an entirely new system 

 of ventral cilia (fig. 2). These new cilia progressively enlarge in 

 length and width, and become closely set, whilst the old cilia 

 so far disappear that, when conjugation is at an end, only 3-4 

 of the anterior and a few of the posterior are extant. At the 



17* 



