102 ORGANIZATION OF THE INFUSORIA. 



Ciliate sections of the Infusoria share between them one typical develop- 

 mental phase, which is usually regarded as an essential factor only of the 

 Metazoa. Among all these organisms the first developmental step exhibited 

 by the simple unicellular reproductive cell or ovum is the division of its mass 

 by the process of segmentation or repeated binary fission, the secondary 

 result of which subdivision is the production of a more or less spheroidal 

 aggregation of simple cells or blastomeres, upon which has been bestowed, 

 in reference to its mulberry-like aspect, the characteristic title of a " Morula." 

 Such a Morula is, however, not limited to the Metazoa. As demonstrated 

 in the chapter devoted to the reproductive phenomena of the Infusoria, 

 certain Ciliata, such as Colpoda, Otostoma, and IchtJiyophthirius, and an innu- 

 merable host of the Flagellata, exhibit in their developmental cycles an 

 essentially corresponding embryonic type. In no point can the segmented 

 cell-mass or Morula of the Metazoon and Protozoon be distinguished, and it 

 is only in the succeeding phases of development that the distinction becomes 

 apparent. Through the disintegration or falling to pieces of the Protozoic 

 Morula, each individual cell or segmented element mostly commences an 

 independent existence, while in that of the Metazoon they remain perma- 

 nently united and initiate new and complex metamorphoses. Exceptional 

 and highly instructive instances among the Protozoa, in which the Morula 

 condition may be said to be retained as the characteristic adult life form, 

 are afforded by the subspheroidal colony-stocks of Magospk&ra, Symira, 

 and Syncrypta, and in a modified manner also by the spheroidal clusters or 

 "ccenobia" of Anthophysa vegetans, Codosiga botrytis, and other sedentary 

 types. The compound Radiolaria, Collosphtzra and Sph&rosoum, &c., may 

 also probably be correctly interpreted as modifications of the moruloid 

 type. Although any direct comparison between the Protozoic and Meta- 

 zoic organisms beyond this Morula stage would be inconsistent, certain 

 most remarkable homoplastic resemblances invite attention. Commencing 

 with the lowest order of the Ciliate group, and selecting as an illustration 

 one of the mouthless Opalinidae, it will be at once recognized that there 

 is here presented a form which, on a simplified and monocellular scale, most 

 distinctly foreshadows or epitomizes the structural type exhibited by the 

 so-called "Planula," or ciliated larva, developed by Metazoic organisms, 

 as the direct outcome of the embryonic Morula already described. Like 

 a typical Opalina, these Planulae are characterized by the possession of a 

 more or less ovate body, which is covered throughout its surface with fine 

 vibratile locomotive cilia. In neither case is there any trace of an oral 

 aperture, so that both animals may be described as closed, externally 

 ciliated sacs. In the Planula this closed sac is composed of multicellular 

 elements, arranged in two distinct superimposed layers, the ectoderm and 

 endoderm, while in Opalina the entire homoplastic counterpart is fashioned 

 out of a single cell. It is a significant fact, however, that in this latter 

 instance the organism has been recently demonstrated to be multi- 

 nucleate, a fact in itself suggestive of latent or potential multicellular com- 



