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IO4ORGANIZA TION OF THE INFUSORIA.affinity.The group in question is that of the Opalinidae. The simplermembers of this section have been already compared with embryonicPlanulae, but in certain of the more highly modified representatives of thefamily the homoplastic resemblance to the similarly endoparasitic tapewormsor Cestoidea is most marvellous. Both are distinguished for theentire absence of an oral or anal aperture, and are in this respect imperforatesaccular bodies. Both occasionally develop horny hooklets or an acetabulateappendage at the anterior extremity wherewith to ensure a permanentadhesion to the internal viscera of the host infested. Both, moreover, sharein common a special mode of reproduction, which, while it recurs among thehigher Annelida, is met with nowhere else among the Protozoa. Referenceis here made to that peculiar form of terminal gemmation exhibited by theOpalina (A noplophryci) prolifcra of Claparede and Lachmann, and severalother allied forms, and in which a long series of buds or segments areproduced 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, thisitself andgroup, including Acineta and its allies, would seem to stand by to present no special homoplastic points of agreement with any Metazoictype. It isproposed here, however, to show that in more respects thanone these suctorial animalcules epitomize most conspicuously, though ona simple unicellular scale, the structural plan of the lower Hydrozoa.To illustrate this resemblance, the Hydroid Polypite in its simplestform, as represented by the so-called " "Dactylozooids recently discoveredby Mr. H. N. Moseley,* to play so important a part in the lifeeconomyof Millepora, Stylaster, and other coral-building Hydrozoa, maybe selected. Such a Dactylozooid or Polypite presents the aspect ofa long, slender, sinuous body, provided with numerous simple or knobbedtentacles, but is entirely devoid of any mouth or stomach. The function ofthese Polypites is simply to seize food and convey it to the mouth-bearingpolypes or " gastrozooids." There can be little doubt, however, that theserudimentary or secondary Polypites represent the primary and independentzooids of some more ancient stock, and the question naturally arises howin such a case did they ingest food ? In reply, it may be submitted that allthat is needed is a perforation of the extremity of each separate tentaculum,such as normally exists in many Ccelenterata, combined with the capacity ofincepting food at these orifices. It is this slight modification, furthermore,that is alone required to produce an organism fundamentally correspondingwith that of an ordinary suctorial Acineta, and whose only means of communicationwith the outer world is similarly through perforationsof theextremities 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).