TO ARTICULATA. 245 



sitions between Protozoa and Articulata is the more difficult to 

 detect because the examples brought forward from the latter 

 group are so simple in their organization as to present a meagre 

 basis of comparison. When you call to mind what has already 

 been said in regard to the marked resemblance between all 

 animals at a very early period of life, you can readily imagine 

 how slight must be the means of distinguishing two diverse 

 kinds which have just begun to lose their general resemblance, 

 and to assume the peculiarities proper to the type to which each 

 respectively belongs. This, in all probability, is the condition of 

 the instance which I shall now lay before you. At first sight 

 this creature, Opalina, (fig. 141,) strikes one as being 

 most decidedly Infusorian. This arises from the 

 fact that it is slightly one-sided, and covered with 

 vibratile cilia. A closer inspection, however, does 

 not reveal the presence of a mouth where one would 

 expect to find it, that is, near the inward curvature <"j 

 at the narrower end, nor at any point. As for the Fig. ui. 

 vibratile cilia, they occur in a large number of animals beside 

 the Protozoa, so that their presence alone cannot have any 

 weight. The so-called contractile vesicle (h) of Opalina is 

 described by the discoverer of this species to " have this pecu- 

 liarity, that, at times, instead of undergoing a total contraction, 

 it constricts itself from point to point in such a way as to trans- 

 form itself into a series of rounded vesicles, disposed, one after 

 the other, like the beads of a rosary." This is a characteristic 

 of the pulsating vessels of a certain class of worms ; and what 

 renders the relationship the more certain is that those worms, 

 which moreover are mostly parasitic and intestinal, have neither 

 mouth nor intestine.* 



There are other species of Opalina, closely allied to this, 



* See the description of Taenia, and the general remarks upon intestinal 

 worms, on pages 79 to 84. 



Fig. 141. Opalina recurva. Clap. 150 diam. A ciliated, infusorian-like 

 worm. 771, the hook-shaped body ; h, the pulsating vessel ; ov, the reproductive 

 organ. From Claparede. 



