IO4 



ORGANIZA TION OF THE INFUSORIA. 



affinity. The group in question is that of the Opalinidae. The simpler 

 members of this section have been already compared with embryonic 

 Planulae, but in certain of the more highly modified representatives of the 

 family the homoplastic resemblance to the similarly endoparasitic tape- 

 worms or Cestoidea is most marvellous. Both are distinguished for the 

 entire absence of an oral or anal aperture, and are in this respect imperforate 

 saccular bodies. Both occasionally develop horny hooklets or an acetabu- 

 late appendage at the anterior extremity wherewith to ensure a permanent 

 adhesion to the internal viscera of the host infested. Both, moreover, share 

 in common a special mode of reproduction, which, while it recurs among the 

 higher Annelida, is met with nowhere else among the Protozoa. Reference 

 is here made to that peculiar form of terminal gemmation exhibited by the 

 Opalina (A noplophryci) prolifcra of Claparede and Lachmann, and several 

 other allied forms, and in which a long series of buds or segments are 

 produced at the posterior extremity, and become successively liberated, 

 like the segments or " proglottids " of an ordinary tape-worm. 



The affinities, real or apparent, of one important section of the Infusoria, 

 that of the Tentaculifera, remain to be discussed. At first sight, this 

 group, including Acineta and its allies, would seem to stand by itself and 

 to present no special homoplastic points of agreement with any Metazoic 

 type. It is proposed here, however, to show that in more respects than 

 one these suctorial animalcules epitomize most conspicuously, though on 

 a simple unicellular scale, the structural plan of the lower Hydrozoa. 

 To illustrate this resemblance, the Hydroid Polypite in its simplest 

 form, as represented by the so-called " Dactylozooids " recently dis- 

 covered by Mr. H. N. Moseley,* to play so important a part in the life- 

 economy of Millepora, Stylaster, and other coral-building Hydrozoa, may 

 be selected. Such a Dactylozooid or Polypite presents the aspect of 

 a long, slender, sinuous body, provided with numerous simple or knobbed 

 tentacles, but is entirely devoid of any mouth or stomach. The function of 

 these Polypites is simply to seize food and convey it to the mouth-bearing 

 polypes or " gastrozooids." There can be little doubt, however, that these 

 rudimentary or secondary Polypites represent the primary and independent 

 zooids of some more ancient stock, and the question naturally arises how 

 in such a case did they ingest food ? In reply, it may be submitted that all 

 that is needed is a perforation of the extremity of each separate tentaculum, 

 such as normally exists in many Ccelenterata, combined with the capacity of 

 incepting food at these orifices. It is this slight modification, furthermore, 

 that is alone required to produce an organism fundamentally corresponding 

 with that of an ordinary suctorial Acineta, and whose only means of com- 

 munication with the outer world is similarly through perforations of the 

 extremities of its prehensile tentacles. In one genus of the Acinetidae, 

 Hemiophrya, it is further worthy of remark that certain of the tentacles only, 

 and those the inner ones, are devoted to the ingestive function, while those 

 * II. N. Moseley, "On the Structure of Millepora," 'Phil. Trans. Roy. Soc.,' vol. clxvii. (1877). 



