ZOOLOGICAL AFFINITIES. 103 



position. In the same way that the Opalinidae may be said to foreshadow 

 the Planula type of organization, it might be suggested that the ordinary 

 stomatode Ciliata, and more especially the multinucleate species, anticipate 

 the next progressive developmental phase of the Metazoic embryo, as 

 represented by the typical "Gastraea" or "Gastrula." The only advance 

 in organization exhibited by this last-named type as compared with the 

 Planula is, that intercommunication between the central cavity and the 

 outer world is now effected through the breaking away of the apical ex- 

 tremity of the primitive closed sac, this apical perforation or " cytostome " 

 constituting the primitive mouth or oral aperture. 



As a rule, the Ciliate Infusorium develops, in addition to an oral orifice, a 

 second or anal aperture, but in many it is not distinctly represented ; in these 

 the conformity to the Gastraea type is necessarily all the more complete. By 

 Professor Haeckel, to whom the scientific world is chiefly indebted for 

 the discovery of the three several "Morula," "Planula," and "Gastrula" 

 developmental phases of the Metazoic series, it has been suggested that, 

 in accordance with the laws of evolution, these successive transitory phases 

 doubtless represent the permanent conditions of as many primitive and 

 pre-existing Metazoic organisms, to which might be attached for convenience 

 the hypothetic titles of " Arche-Morula," " Arche-Planula," and "Arche- 

 Gastrula." From the foregoing data it is very evident, however, that such 

 archetypes are even yet in existence, and may be successfully sought for 

 among the representatives of the Protozoa. 



Entering now upon an examination of those few instances in which 

 representatives of the Infusoria appear to pre-typify, not embryonic or 

 transient, but fully matured conditions of certain lower Metazoa, attention 

 may be first directed to those forms, such as Paramecium and Ophryoglena, 

 in which the subcuticular layer abounds with the minute evertile structures 

 known as trichocysts. As already pointed out many years previously by 

 Kolliker, Oscar Schmidt, and other authorities, the correspondence between 

 such types and certain of the lower Annelida or Turbellaria is, so far as 

 the general form and the possession and disposition of their trichocysts is 

 concerned, most conspicuous, so that on a simple monocellular scale these 

 Infusoria may be said to foreshadow or pre-typify these simple Annelids. It 

 has been suggested by Louis Agassiz, and also by Diesing, that the typical 

 Vorticellidae, with their closely approximated oral and anal apertures and 

 well-developed pharyngeal tube, indicate a considerable conformance with 

 the fundamental organism of the Polyzoa the last-named authority, indeed, 

 transferring this section of the Infusoria to that series. In its originally 

 implied sense it is necessarily impossible to maintain such a proposed 

 affinity, but in a more remote manner, regarding such animalcules as 

 modifications of unicellular zooids in the direction indicated, it may perhaps 

 be accepted. While, as already intimated, Paramecium and its allies 

 would appear to pre-typify the Turbellaria, another more lowly organized 

 group of the Ciliata exhibits an entirely distinct and highly interesting 



