GENUS OP A LIN A. 56 1 



Opalina dimidiata, Stein. Pl. XXVI. Figs. 16-18. 



Body elongate, irregularly ovate, flattened, widest anteriorly, tapering 

 and more or less pointed posteriorly, three or four times as long as broad ; 

 the right border of the anterior extremity produced outwards in a keel-like 

 manner, the left. border evenly arcuate throughout ; striations of the cuticular 

 surface straight and slightly oblique on the dorsal surface, arcuate on the 

 ventral one, entirely clothed with long, fine, matted cilia ; no contractile 

 vesicle ; endoplasts multiple, inconspicuous, first appearing in the young 

 zooids as a single central spherical body, afterwards breaking up by repeated 

 segmentation into innumerable smaller spherules, which become dispersed 

 through the substance of the parenchyma ; increasing by longitudinal and 

 transverse fission. Length of adults, I-75" to 1-50". 



Hab. — Intestine and rectum of the Edible Frog, Rana esailenta, and Biifo 

 cinereiis. 



This species, as pointed out by Zeller, is identical with the form figured and 

 described by T. W. Engelmann* under the title of Opalina ranan/m, but from which 

 it differs essentially both in the more attenuate contour of the adult zooids and in the 

 distinct hosts with which it is found associated. Overlooking this slight matter 

 of mistaken specific identity, the credit must be awarded to Engelmann of contri- 

 buting the earliest data of importance with reference to the structural and develop- 

 mental history of the Opalin(z. Previous to his researches connected with the 

 present species, the existence of an endoplast was entirely denied to the represen- 

 tatives of the genus ; the development of the animalcules from the minute encysted 

 bodies, as here described, was likewise unknown, while Zeller, taking as a basis and 

 following up with advantage the results of Engelmann's investigations, has left but 

 little to be added to our knowledge of their entire life-cycle. Engelmann's report of 

 the developmental phenomena of this Opa/ina dimidiata may be thus summarized : 

 On examining frogs in their larval or tadpole state, at a period when their bodies 

 measured a little more than a quarter of an inch in length, their intestinal tracts 

 were found to contain numerous spherical transparent cysts having a diameter of 

 about the i-2oooth part of an English inch. The riper of these contained a small, 

 elongate, ciliated body, curled upon itself and almost filling the cavity of the cyst. 

 Numerous elongate ciliate bodies, identical with those contained in the cysts, were 

 swimming freely in the surrounding fluid, presenting all the characteristics of typical 

 Opaline?, but of smaller size and inuch more attenuate proportions even than the 

 adult examples. These free-swimming larval zooids, see PI. XX VI. Fig. 18, 

 are described as highly flexible, of elongate-lanceolate contour, with an attenuate 

 and tail-like posterior extremity; the cuticular surface is finely striate longitudi- 

 nally, and covered with a dense clothing of very fine cilia, which are often pro- 

 duced posteriorly as a brush-like tuft. In both the free-swimming and in the cyst- 

 enclosed animalcules a single, central, spherical endoplast of considerable relative 

 size was, while clearly visible under ordinary conditions, rendered still more conspi- 

 cuous by the addition of dilute acetic acid. Investigating more matured specimens 

 of the frogs' larvae, it was found that Opalince of gradually increasing size and stouter 

 proportions were contained within them, yielding at length examples in no wavs 

 distinguishable in either shape or size from the ordinary adult zooids of Opalina 

 dimidiatuni. Conjointly with this increase in the dimensions of the animalcules it 

 was further found that the endoplast or nucleus presented a progressively compound 

 character. Though single or simply spherical in the youngest individuals, it con- 

 sisted of two similar spheroidal bodies in rather older ones ; of four, eight, and 

 sixteen such bodies in still further developed zooids ; while in the full-grown and 



* ' Morphologische Jahrbuch,' Bd. I, Heft 4, 1876. 



