GENUS OPALINA. 559 



plast, a new and independently developed endoplastule subsequently making its 

 appearance in the other moiety. The members of the genus Opalina proper, as at 

 present known, are parasites exclusively of the intestinal viscera of various tailed 

 or tailless Amphibia. 



Opalina rananim, Purk. 

 Pl. XXVI. Figs. 1-9, and 20, and Pl. XXXI. Fig. 19. 



Body broadly ovate or elliptical, flattened, scarcely twice as long as 

 broad, evenly rounded posteriorly, the anterior extremity bluntly pointed ; 

 the anterior half of the right-hand border projecting in a keel-like riianner, 

 the opposite one evenly arcuate ; the cuticular surface finely striate through- 

 out, the striation on the dorsal surface taking a rectilinear-oblique course, 

 and on the ventral one an undulate or arcuate direction ; endoplast single 

 in the youngest animalcules, multiple in the adult zooids ; parenchyma 

 enclosing numerous minute, refringent, discoidal corpuscles ; increasing by 

 oblique or transverse fission. Length 1-45" to 1-30". 



Hab. — Intestine and rectum of the common frog, Rana temporaria, and 

 toads, Bitfo variabilis et cinereiis. 



This, the largest and most widely known representative of the genus Opalina, while 

 receiving its characteristic title from Purkinge and Valentin in the year 1835, was 

 figured and described by J. A. E. Goze under the name of the " Flimmerwalzen " or 

 " Flimmerquadrate " so long since as the year 1782. Through the recent investiga- 

 tions of Zeller we are now made familiar with the entire reproductive and develop- 

 mental cycle of this form, and with the various modifications undergone by it in its 

 onward growth from the embryo to the adult state. The minute cysts of this animal- 

 cule, Pl. XXVI. Fig. 6, were abundantly met with by Zeller in the rectum and excreta 

 of Rana temporaria during the earlier months of the year, and on being placed 

 in vessels with the tadpoles or frog-larvte were abundantly devoured by the young 

 amphibia during the inception of their customary food. Within the intestinal tract of 

 their new host the wall of the cyst is ruptured, releasing the contained embryo. On 

 first making its exit from its transparent capsule, Pl. XXVI. Fig. 7, the animalcule is 

 of an ovate shape, slightly flattened, with a rounded and widest anterior end and 

 pointed posterior extremity, the proportions being exactly the reverse of wliat is met 

 with in the parent form. Centrally, the parenchyma encloses a single conspicuous 

 spheroidal endoplast, and has, dispersed through its substance, an indefinite number of 

 the characteristic discoidal refringent corpuscles. Within a short interval, Pl. XXVL 

 Fig. 8, the body assumes a more elongate shape, is curved anteriorly slightly 

 towards the right, while the endoplast becomes divided into two or four equal sphe- 

 roidal portions. As growth proceeds a still more attenuate shape is arrived at, the 

 endoplast meanwhile multiplying within the parenchyma by progressive segmenta- 

 tions. In the most characteristic stage of its development the young Opalina 

 rananim presents an elongate-lanceolate contour, with a prominent keel-like anterior 

 border that agrees closely with the normal adult state of Opalina dimidiata and 

 Opalina similis, as delineated at Figs. 16 and 19 of the same plate. The length of 

 the animalcule at this epoch of its existence is equal to four or five times that of its 

 greatest breadth, but it now begins to diminish these proportions and to increase in 

 width. Widening out until the length scarcely exceeds twice its breadth, the 

 posterior extremity still retains its narrower and pointed outline, and the right 

 anterior angle its projecting keel. In this stage its contour corresponds remarkably 

 with that of the adult form of Opalina obtrigona, as illustrated by Pl. XXVL Fig. 10. 

 The posterior region of the body commences now to fill out, and becoming wider 

 and more rounded than the anterior or apical extremity, the normal shape and 

 size of the adult zooid, as represented by Pl. XXVL Fig. i, is ultimately attained. 



