326 M. M. Mercaur 
both marine and frish-water Annelids”. He says “the simple struc- 
tureless body of these first named parasites has really very little 
in common with Opalina, properly so-called — an abundance of highly 
refringent granules being the only differentiated portions of its 
substance, no trace of the nucleus and contracted vesieles, nor of 
the furrowed cuticle of true Opalina being observable. It is not 
impossible that these swimming flakes of sarcode — for they are 
nothing more — may undergo subsequent metamorphosis of the most 
extreme character.” 
ENGELMANN published in Dutch (1875) and in German (1876) the 
first account of the growth of the young Opalinas in the rectum 
of the tadpole. He reared tadpoles from the egg in glass dishes. 
He does not say how they were fed or how they became infected, 
though he mentions remnants of plants as present in their alimentary 
canals. [Probably the tadpoles were naturally infected from the 
material in the dishes in which they were kept, else ENGELMANN 
would have mentioned artificially infecting them from the material 
in the recta of the frogs.| He found uninucleated cysts in the recta 
of the tadpoles and he also describes stages in the development 
of the little Opalinas from the uninucleated to the multinucleated 
condition. [His figures suggest tha the may have seen both micro- and 
macrogametes, though he did not recognise them as such, or observe 
copulation. ENGELMANN says he studied O. ranarum from tadpoles of 
Rana esculenta. As this parasite has never before or since been 
reported from this host it seems possible that cross infected material 
was studied. The minute Opalinas figured seem clearly to be 
O. ranarum, for many of them are not slender enough for the 
corresponding stages of O. dimidiata'), while they resemble ZELLER’s 
figures of the minute O. ranarum in the tadpoles. His observation 
of uninucleated cysts suggests either that possibly ENGceLMANN saw 
encysted zygotes, as NERESHEIMER thinks, or that infection cysts of 
O. intestinalis or O. caudata may have been present also, for uni- 
nucleated infection cysts are common only in these two of the 
Europaean Opalinae though they are not rare in O. ranarum (ef. 
p. 281). As the tadpoles were reared from eggs, there can be little 
doubt of their correct identification, since the time of year when 
the eges were found, and their size and color, as well as the size 
and color of the tadpoles themselves, would distinguish them from 
") NeresHermer (1907) thinks that they were O. dimidiata, but I have not 
found the minute individuals of this species. presenting this appearance. 
