OF THE PROTOZOA.— opalin.5:a. 269 



has evidently seen a specimen which has very recently completed the act of 

 self-fission and not yet reacquired its rounded posterior extremity. The 

 dark stiipe shown at the fore part, and supposed to indicate a mouth, repre- 

 sents the uncinate apparatus above described. Stein would call this form of 

 Opalina the 0, armata, and regard it as a further stage of development of 

 his so-called 0. Lumhrici. 



This view is supported by the fact that he has never met with j^oung 

 individuals of 0. armata ; for all the specimens he encountered were of a nearly 

 equal size, and larger than the largest of 0. Lumhrici, in company with 

 which young beings are very common. Thus 0. armata attains a length of 

 1-12'" to 1-9'", and 0. Lumhrici of not more than 1-14"' ; even the products 

 of fission of the former are from 1-16"' to 1-14'". 



** If now it be considered that, excepting the horny process, not the least 

 difference in structure exists between 0. Lumhrici and 0. armata, it is ren- 

 dered very probable that the latter is merely a fui'ther stage of development 

 of the former. If this be the case, a subsequent more considerable meta- 

 morphosis of 0. armata may be presumed, when it becomes transferred to a 

 more favourable habitat, as happens when the worm it inhabits becomes the 

 food of some other animal. I have not actually seen Opalina armata adhering 

 to the surface of the intestine, for I have always found it amidst the undi- 

 gested mineral and organic fragments which fill the alimentaiy canal of the 

 earth-worm. Hence it is more likely that the adhesive organ is destined to 

 subsequently fix the Opalina in a more permanent manner." 



The long pulsating vessel seen in Opalina Planariarum and in 0. uncinata 

 deserves particular notice, by reason of its peculiaiity. Stein has described 

 it in the first-named species, where it extends the entire length of the ani- 

 malcule, as bounded by a definite, delicate, structiu'eless membrane, and to 

 be without the outlets Schultze imagined. It contains a clear liquid like 

 water, which, by its rhythmical movements, it forces to and fro -within it. 

 On killing the animal with alcohol, the walls of the vessel are rendered very 

 evident. It becomes divided through the centre in the act of self- fission, and 

 is, in Stein's opinion, not homologous with the contractile vesicles of the 

 Ciliata. 



Ntjclfxs. Self-divtsiox. Supposed Embryo. — The nucleus is a very evident 

 organ in all the Opalincea, with the single exception of 0. Ranarum, in which 

 Stein has sought for it in vain among multitudes of specimens and by the aid 

 of various reagents. In this same exceptional species it is also to be noted 

 that he never vidtnessed the act of fission, yet Siebold (" Ueher Monostomum,'' 

 Wiegmann's Arcliiv, 1835) described, in an Opalina living as a parasite in 

 the intestines of a frog, the existence of a number of smaU embryos within 

 a ca\'ity of the posterior extremity of the body : whether this animalcule, 

 however, was the Opalina Ranarum does not appear ; for the peculiar habitat 

 does not by any means prove such to be the case. 



A contrast occurs in the Opalina Branchiarum, where the nucleus which 

 lies in the axis of the body has the same figure as the entire being, and one- 

 half its dimensions. Even among examples of the same species the position 

 of the nucleus varies exceedingly. Simultaneously with the appearance of a 

 constriction in the general figure, the nucleus shows signs of approaching 

 fission ; but ere this is manifested it assumes a central position (whatever may 

 have been its previous one), so that each of the two future segments may 

 acquire an equal section of it. Moreover, it would appear, in some cases at 

 least, that the constriction and scission of the body advance more rapidly on 

 one side than on the other of the animal. 



According to Stein, the production recorded by Schultze (Beifrdge zur Natur- 



