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102 ORGANIZATION OF THE INFUSORIA.Ciliate sections of the Infusoria share between them one typical developmentalphase, which is usually regarded as an essential factor only of theMetazoa. Among all these organisms the first developmental step exhibitedby the simple unicellular reproductive cell or ovum is the division of its massby the process of segmentation or repeated binary fission, the secondaryresult of which subdivision is the production of a more or less spheroidalaggregation 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 demonstratedin the chapter devoted to the reproductive phenomena of the Infusoria,certain Ciliata, such as Colpoda, Otostoma, and IchtJiyophthirius, and an innumerablehost of the Flagellata, exhibit in their developmental cycles anessentially corresponding embryonic type. In no point can the segmentedcell-mass or Morula of the Metazoon and Protozoon be distinguished, and itis only in the succeeding phases of development that the distinction becomesapparent. Through the disintegration or falling to pieces of the ProtozoicMorula, each individual cell or segmented element mostly commences anindependent existence, while in that of the Metazoon they remain permanentlyunited and initiate new and complex metamorphoses. Exceptionaland highly instructive instances among the Protozoa, in which the Morulacondition may be said to be retained as the characteristic adult lifeform,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 sedentarytypes. The compound Radiolaria, Collosphtzra and Sph&rosoum, &c., mayalso probably be correctly interpreted as modifications of the moruloidtype. Although any direct comparison between the Protozoic and Metazoicorganisms beyond this Morula stage would be inconsistent, certainmost remarkable homoplastic resemblances invite attention.Commencingwith the lowest order of the Ciliate group, and selecting as an illustrationone of the mouthless Opalinidae,it will be at once recognized that thereis here presented a form which, on a simplified and monocellular scale, mostdistinctly foreshadows or epitomizes the structural type exhibited by theso-called "Planula," or ciliated larva, developed by Metazoic organisms,as the direct outcome of the embryonic Morula already described. Likea typical Opalina, these Planulae are characterized by the possession of amore or less ovate body, which is covered throughoutits surface with finevibratile locomotive cilia. In neither case is there any trace of an oralaperture, so that both animals may be described as closed, externallyciliated sacs. In the Planula this closed sac iscomposed of multicellularelements, arranged in two distinct superimposed layers, the ectoderm andendoderm, while in Opalina the entire homoplastic counterpart is fashionedout of a single cell. It is a significant fact, however, that in this latterinstance the organism has been recently demonstrated to be multinucleate,a fact in itself suggestive of latent or potentialmulticellular com-