480 



CLASS II, CILIA TA. 



Nauplian Crustaceous lame, are characterized by the possession of only a single 

 median visual organ. Many of the higher Hypotrichous Ciliata, e. g. Stylonychia 

 and Euplotes, would finally, by virtue of their not unfrequently branched setose 

 appendages, appear to exhibit a latent Arthropodous bias. 



With the aid of the evidence submitted here and in the preceding volume, it is 

 finally found possible not only to predicate the probable lines of evolution by which, 

 out of the Holotrichous, Peritrichous, and Hypotrichous orders of the Infusoria Ciliata, 

 all of the more important Metazoic groups have been evolved, but also to produce a 

 genealogic scheme, see p. 479, indicating an unbroken chain of animal organization 

 of gradually increasing complexity from the lowest Rhizopod up to the Vertebrata. 

 The broad and seemingly almost insuperable hiatus which, failing the group of the 

 Spongida, seemed to intervene between the Protozoic and Metazoic series, need no 

 longer present any substantial obstacle to the taxonomist. For while the three 

 foregoing orders of the Ciliata still remain undoubted and easily recognized 

 Protozoa, it as evidently needs but the interposition of some innate force, akin to that 

 of crystallization, to transform their plastic and comparatively amorphous protoplasmic 

 bodies into mullicellular aggregates or Metazoa. Nay more, numberless examples 

 of the Ciliata are multinucleate, and therefore potentially multicellular, and furnish, 

 by reason of such differentiation, the most perfect possible links of connection 

 between the Protozoic and Metazoic series. Where, consequently, as in the several 

 cases here submitted, these Protozoic organisms are more or less perfect isomorphs 

 of the mature or larval phases of Metazoic structures, it may be consistently 

 predicated that they are phylogenetically connected with them. 



A beautiful and highly instructive illustration of the manner in which a unicellular 

 and simply nucleated protoplasmic body may pass first into a multinucleate and then 

 into a multicellular condition is afforded by the figures given by Metschnikoff,* 

 copied in F. M. Balfour's ' Comparative Embryology,' and herewith reproduced, of 

 the segmentation of the ovum of the Rose Aphis, Aphis rosce. Fig. 3 in this series is 



Figs, i to 4, illustrating four successive phases of the segmentation of the ovum of A $ 'his rusts. A granular central 

 yolk mass and an outer protoplasmic layer are distinctly visible in every instance. In Figs, i and 2, two and four 

 nuclei have respectively appeared within the outer protoplasm. In Fig. 3 a number of nuclei have arranged themselves 

 at regular distances throughout this region, while in Fig. 4 the protoplasmic layer has become divided into a number of 

 columnar cells, which correspond with the nuclei (after Metschnikoff). 



Fig. 5, an adult example of Dicyema iypus, whose histologic composition accords closely with that exhibited by the 

 aphis ovum represented in the preceding Fig. 4 ; ax, central nucleated axial cell ; ec, multicellular ectodermal layer 

 (after Ed. Van Beneden). 



more especially noteworthy, since it corresponds with a typical Opalina and depicts 

 that multinucleate and potentially multicellular state that is definitely assumed in 

 the succeeding phase as delineated at Fig. 4. Metazoa whose adult structure 

 may be said to be scarcely in advance of what obtains in the simplest multicellular 

 condition of the ovum delineated at Fig. 4," are found embodied in the two 

 recently discovered orders of the Dicyemida and Orthonectida. Both of these 

 consist of mouthless, elongate ovate, more or less completely ciliated, essentially 

 endoparasitic organisms, the former as yet having been obtained infesting exclusively 

 the renal organs of various Cephalopoda, while the latter more extensively distributed 



* " Embry. Stud. Insecten," 'Zeit. Wiss. Zool.,' Bd. xvi., 1866. 



