562 ORDER HOLOTRICHA. 



perfectly matured representatives of the species, this primarily simple endoplast 

 had resolved itself into innumerable spheroidal fragments. 



Although demonstrating the existence and extensive distribution of the endo- 

 plastic system in this species, and the development of the adult forms from the 

 minute encysted bodies, Engelmann has contributed nothing further towards its 

 reproductive history, though he was inclined to infer that the minute encystments 

 first observed were derived immediately from the ultimate spheroidal segmentations 

 of the endoplast of the fully matured zooids. This hiatus in the developmental 

 cycle of the type now under consideration has been successfully filled in by Zeller, 

 who has shown that the fully grown animalcules break up by continuous fission in 

 the manner already described of Opalina ra?iarum, the ultimate fragments forming 

 similar encystments. A slight variation in this process of fission is exhibited by 

 O. dimidiatum, the subdivision being accomplished alternately in a longitudinal and 

 transverse direction instead of in an oblique and transverse one, as occurs in the 

 preceding type. In the latter instance it frequently happens that the two separated 

 portions are of very unequal dimensions, the posterior moiety being considerably 

 the smaller. This unevenness of the calibre of the segmented moieties recurs, as 

 presently shown, in a yet more conspicuous form among certain representatives of 

 the allied genus Anoplophrya, 



Opalina obtrigona, Stein. Pl. XXVI. Figs. 10 and ii. 



Body flattened, subtriangular, about twice as long as broad ; the anterior 

 border considerably the widest, tapering thence gradually backwards to- 

 wards the attenuately pointed posterior, the right margin convex, the left 

 one concave; cuticular striations taking a rectilinear-oblique course on 

 the dorsal surface, and an arcuate direction on the ventral one ; endoplasts 

 numerous, spheroidal, dispersed throughout the substance of the paren- 

 chyma ; parenchyma enclosing also minute discoidal corpuscles ; segmen- 

 tation longitudinal and transverse. Length 1-50" to 1-40". 



Hab. — Intestine of the Tree- Frog, Hyla Etivopcea. 



The normal adult contour of this species closely resembles that of the half- 

 grown condition of Opalina ranarum, and also the anterior zooid produced by 

 the first act of fission of that type. The development of the ordinary zooids 

 from cyst-like bodies resulting from the minute subdivision of the adult animalcules, 

 as also the multiplication of the species by simple fission, has been observed by 

 Zeller. The formation of cysts takes place in the spring months, April and May, 

 when the frogs are spawning and lead a temporary aquatic life. Being then cast 

 out with the ordinary excreta into the surrounding water they are there devoured by 

 the young tadpoles, and begin anew their cycle of development. 



Opalina intestinalis, Ehr. sp. Pl. XXVI. Fig. 19. 



Body identical in form with Opalina dunidiata, but smaller in size, en- 

 closing a single ovate endoplast only, or two such endoplasts situated 

 toward the anterior extremity of the body, and united to one another by a 

 thread-like commissure or funiculus ; dividing by longitudinal and transverse 

 fission. Length i-ioo" to 1-75". 



Hab. — Intestine oi Pelobates fuscus and Rana esculenta. 



This species, upon which Ehrenberg conferred the name of Btirsarla mtcstuialis, 

 represents one of the oldest known infusorial forms, it finding a place among the 

 text and figures of Leeuwenhoek's ' Opera Omnia,' p. 49, fig. a, 1683, under the 



