CLASS II. CILIATA. 475 



Before entering upon a systematic description of the Ciliata, it is worthy of 

 remark that until within a comparatively recent date the assemblage of organisms 

 included within this class were accepted by many eminent authorities as possessing 

 the only sound claim to the title of the Infusoria. The Flagellata, previous to 

 their more exhaustive scrutiny by recent investigators, were regarded for the most 

 part as Protophytes or the so-called zoospores of higher Cryptogamic plants, while 

 the Tentaculifera or Acinetidae were pronounced to be embryonic conditions only 

 of various Peritricha. Undoubtedly, this section of the Ciliata is one of the 

 largest and most important groups of the Protozoic sub-kingdom. In no other of the 

 equivalent subdivisions is histologic differentiation carried to such a marked extent ; 

 many of the higher forms being indeed so modified in this respect that they may 

 be unhesitatingly cited as furnishing the most complex expression of unicellular 

 organization. To the notice of the evolutionist, to an equal or even greater degree 

 than to the histologist, the Ciliate Infusoria deservedly commend themselves. It is 

 beyond question to the ranks of this extensive class that attention may be profitably 

 directed in the search after those ancient stock-forms or archetypes out of which 

 several of the more important groups or phyla of the Metazoa have been gradually 

 evolved. That the group of the Spongida does not, as maintained by some 

 contemporary authorities, furnish the long sought for interconnecting link between 

 the Protozoic and Metazoic series is substantially supported by the evidence 

 submitted by the author in Chapter V. of the preceding volume, in which it is 

 shown that, notwithstanding the plausibility and fascination of external appearances, 

 these organisms remain in all structural and developmental details thoroughgoing, 

 though peculiarly modified, Protozoa. 



Already, in vol. i., p. 103 et seq., the homoplastic resemblances and possible 

 biogenetic relationship that subsist between the Ciliate and other Infusoria and the 

 Metazoic animal series have been discussed at some length. Further time devoted to 

 the consideration of this important subject, enables the author to greatly extend 

 the scope of the comparisons previously instituted, and to submit, in connection 

 with the woodcut illustrations on p. 477, and succeeding Genealogical Table, a 

 possible clue to the phylogeny of all of the more important Metazoic groups. 

 That the Holotrichous Ciliata may be consistently regarded as the archetypes 

 of the lower worms, has been frequently advocated, the likeness between the former 

 and the simpler Turbellaria being so marked, see woodcut, Figs. 3 and 4, that 

 many earlier authorities have proposed to include these Infusoria in the Annelidous 

 class ; K. M. Diesing, more especially, in recognition of such likeness, established 

 for their reception the new order of the " Prothelmintha." As related in vol. i. p. 26, 

 this authority included, however, in this order not only the Holotrichous Ciliata, 

 but, with the exceptions of the Vorticellidae and Stentoridae, all the Ciliate and 

 Flagellate animalcules. In addition to the resemblances subsisting between the 

 Holotrichous Infusoria and the Turbellaria, as manifested by their minute size and 

 corresponding form, by their entire and even ciliation, by their common possession 

 of trichocysts, and by the development in the members of the last-named group of 

 a water-vascular system, which may be regarded as a modification of the contractile 

 vesicles of the Infusoria, it may be mentioned that many of the Rhabdoccelous 

 Turbellarians multiply as do the Holotricha and other Infusoria by the simple 

 process of transverse fission. Taken collectively, the numerous collateral charac- 

 teristics cited yield almost overpowering evidence in favour of the biogenetic 

 relationship that is here advocated.* 



Other Metazoic groups, in addition to that of the Turbellaria, would seem to 

 possess a substantial claim to an Holotrichous ancestry. In the sub-kingdom 



* The near approach of the Ciliate Infusoria to the structural plan of the lower Turbellaria is 

 briefly indicated by Professor Huxley at p. 678 of his 'Anatomy of Invertebrated Animals,' ed. 

 1877; while, in a communication addressed to the author since the publication of the first two parts 

 of this Manual, this authority has more distinctly referred to the Holotricha as probably constituting, 

 in connection with the Turbellaria, the proper line of evolution from the Protozoic to the Metazoic 

 series. The Spongida are further alluded to as occupying in such case, with reference to the 

 Metazoa, a position equivalent to that held by the Tunicata in relation to the Vertebrata. 



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