CLASS II. CILIATA. 481 



group affects the intestinal viscera and surrounding tissues of various Nemerteans, 

 Turbellaria, and Ophiurida. In the case of Dicyema, see Fig. 5 of the accom- 

 panying woodcut, histological differentiation, as attained in the adult, takes the form 

 of a single peripheral cellular layer, within which is enclosed a single long cylindrical 

 or subfusiform axial cell. With the Orthonectida, Rhopalura, a slightly more 

 complex structural formula is introduced through the subdivision of the central 

 axial cell into cellular elements. The external resemblance that subsists between 

 Dicyema and certain ciliated Infusoria was held by Claparede and Lachmann to be so 

 close, that these authorities figured and described one species, D. Muelleri, in their 

 ' Etudes sur les Infusoires,' as a type whose nearest affinities were probably to be 

 sought for amongst the Opalinidae. As constituting a stage in advance of these 

 Ciliata, equivalent to that held by Fig. 4 with reference to No. 3 of the segmentation 

 phases of the aphis ovum here delineated, this interpretation of Dicyema may perhaps 

 be not inconsistently maintained. It might be further suggested, with reference 

 to such affinity, that the single axial cell, the sole representative of the hypoblastic 

 element or entoderm in Dicyema, is to some extent foreshadowed in the remarkably 

 prolonged and correspondingly axial so-called nucleus, as developed in the more 

 abnormal Opaline genera Anoplophrya and Hoplitophrya. In recognition of the 

 essentially intermediate structural position with reference to the Protozoa and 

 typical Metazoa, that is occupied by the Dicyemida, Ed. Van Beneden* has proposed 

 to assign it to an altogether distinct and intermediate sub-kingdom, that to be 

 distinguished by the title of the " Mesozoa." Professor Huxley t is likewise 

 inclined to support this view, though with reference more to the remarkable 

 developmental phenomena. In so far as the germs in Dicyema are produced from 

 within the single axial cell, such reproductive cell is correlated by this authority 

 with the central " capsule " of a Radiolarian ; while the peculiar manner in which 

 the peripheral cells in the embryo grow round and partly enclose the axial one, 

 he maintains, corresponds with the phenomenon of " epiboly " as it occurs in many 

 ordinary Metazoa. 



The embryological evidence so far at disposal, as now submitted and indicated in 

 the accompanying table, unquestionably points to the derivation of the Ccelenterata, 

 Turbellaria, Chaetopodous Annelida, and Scolecida from a Holotrichous archetype ; 

 the embryos in examples of all these Metazoa being completely clothed with fine 

 vibratile cilia, and scarcely distinguishable from Holotrichous Infusoria. In a second 

 series, including more especially the Polyzoa, Mollusca, Echinodermata, typical 

 Rotifera, and many Polychsetous Annelida, the larval forms are characterized almost 

 exclusively by their Peritrichous plan of ciliation and remarkable conformance with 

 certain members of the Peritrichous Infusoria ; while in the third Ciliate group, that of 

 the Hypotricha, there is an apparent modification in the direction of the Rotifera, and 

 through them to the Arthropoda. Taken collectively, it is clearly shown that the 

 peritrichously ciliated larval form represents a preponderating factor in the onto- 

 genetic history of the Metazoic series ; its protozoic isomorphs, as typified by a 

 Peritrichous Infusorium, being, in accordance with the fundamental laws of evolution, 

 the archetype or stock-form from whence the principal Metazoic groups were primarily 

 derived, and out of which, as shown in the accompanying table, the genealogy even 

 of the Vertebrata may with facility be defined. Though the biogenetic scheme here 

 introduced is mainly tentative, and necessarily imperfect at many points, it may 

 be confidently anticipated that a more extended acquaintanceship with the Ciliate 

 Infusoria and initial or larval phases of the Invertebrata will be productive of data 

 permitting of a yet closer and more perfect amalgamation of the Protozoic and 

 Metazoic groups. In many instances, through the epitomization and abbreviation 

 of their developmental course, the clue sought for is dim or even altogether 

 obscured, but in other cases it is manifested with an amount of perspicuity that can 

 scarcely escape the recognition of the earnest interpreter of Nature's problems. 



* " Recherches sur les Dicyemides," ' Bull, de 1'Acad. Roy. de Belgique,' torn. xli. No. 6, and 

 torn. xlii. No. 7, 1876. 



t ' Anatomy of Invertebrated Animals,' 1877. 



