558 ORDER HOLOTRICHA. 



Annelidous and Infusorial classes is deemed sufficient. An altogether distinct 

 explanation, however, may be tentatively submitted for the consideration of those 

 who are disposed to concede to them an independent origin. In this connection 

 attention may be more especially directed to that correlation of an Opaline 

 animalcule with an undeveloped Metazoic ovum that has been instituted at page 

 480, previously quoted. As an afterthought to such correlation, the author has 

 been impressed with the idea that the Opalinida may actually have originated from 

 the ova of the respective hosts which they are found inhabiting ; these ova, failing 

 the vital force requisite to carry them through the later segmentation phases 

 and thence to the parent form, having yet possessed a sufficient store of such 

 force to permit of their prolonged existence as independent beings able by the 

 sim2:)le process of fission to reproduce their kind. The very facts of these remark- 

 able organisms being without exception mouthless, and without exception endo- 

 parasitic, might be quoted as evidence in favour of this interpretation. The fact, 

 again, that side by side with the mouthless Opalitice, as found in the common frog, 

 there subsists a ciliated Infusorium, Nydot/ierus, possessing not only an oral but also 

 a highly differentiated anal aperture, militates against the consistent acceptance of 

 the former as degenerate scions of the ordinary Ciliate stock, and favours, on the 

 other hand, the alternative of their independent derivation. 



The several genera into which this family group has been recently subdivided 

 by Professor Stein are readily distinguished by the characters afforded by the 

 presence or absence of contractile vesicles and by the nature of the supplementary 

 prehensile organs developed at the anterior extremity. The locomotive appendages, 

 or cilia, in the group of the Opalinid^e, demand brief notice. Among the majority 

 of the forms included, they are distinguished from those of the more ordinary 

 Ciliata by their conspicuously matted or tufted aspect ; this peculiarity is, nevertheless, 

 shared by various other endoparasitic Infusoria, such as the genera Conc/iop/it/iiriis, 

 Balatitidium, and Nydotherus, and doubtless fits them for locomotion in the denser 

 fluid medium which they inhabit. 



Genus I. OPALINA, Purkinge. 



Animalcules mouthless, free-swimming, ovate or elongate ; cuticular 

 surface delicately striate ; evenly ciliate throughout, furnished with no 

 supplementary organs of prehension ; endoplast spherical or ovate, single in 

 the youngest individuals, usually breaking up by repeated segmentation, 

 as growth proceeds, into innumerable minute, rounded, nucleolar bodies, 

 having a clear peripheral zone and a central nucleolus or endoplastule ; 

 no contractile vesicle. Entirely endoparasitic. 



In accordance with the results of the latest investigation of the members of this 

 genus, as obtained by Ernst Zeller, the external surface of Opalina is not suffi- 

 ciently indurated to form a distinct and continuous cuticle, being composed of 

 numerous closely approximated, elastic, finely granular, muscular fibrillar, which 

 remain bound together during the life of the animalcule, but fall apart, as shown at 

 PI. XXVI. Fig. 20, on being treated with dilute acetic acid. It is the interstices or 

 lines of juncture between these fibrillar that impart to the Opaluice their character- 

 istic striated aspect. In the abnormal multiplication of the endoplast by duplication, 

 as manifested in the adult zooids, the representatives of this genus differ not only 

 from the remaining genera of the Opali/iidce, but from all ordinary Ciliata. The 

 method of separation of each endoplastular fragment, as reported by Zeller, exhibits 

 an adi.litional peculiarity. The simply rounded form of the primitive single endo- 

 plast, under such conditions, is exchanged for a more or less elongate-ovate outline, 

 and is eventually separated into two equal halves through a central constriction ; the 

 original, minute, central, granular, endoplastule-like body or nucleolus does not, how- 

 ever, take part in this division, but remains intact in one half of the divided endo- 



