478 CLASS IT. CILIATA. 



Atrochal formula.* It is a further significant fact that in this type a fascicle of long 

 whip-like cilia is developed from the oral region, which may be regarded as 

 homologous with the terminal tuft common to both the Atrochal and Mesotrochal 

 Metazoic larvae. A yet more interesting and significant modification of the Telo- 

 trochal plan is perhaps exhibited by the recently discovered Peritrichous Infusorium 

 represented by Fig. 8 of the accompanying woodcut, and upon which, with reference 

 to the likeness indicated, the author has conferred the generic name of Tclotrochidium. 

 While somewhat resembling at first sight a temporarily detached Vorticella with a 

 supplementary posterior ciliary circlet, it differs fundamentally from such a type in 

 that the anal aperture does not open, as with the typical Vorticellidje, upon the oral 

 vestibulum, but at the posterior extremity of the body and to the rear of the hinder 

 circlet of cilia. The anal passage following upon the aperture is conspicuously 

 visible for some distance within the surface of the cuticulum, and, as will be at once 

 recognized, it requires but a further prolongation and juncture of the oral and anal 

 passages to produce an organism indistinguishable from Gegenbaur's representation of 

 a Telotrochous Annelid larva, reproduced at Fig. 7. 



A remaining and very important section of the Metazoa, with which as yet no 

 attempt has been made to demonstrate an infusorial phylogenetic origin, is that of 

 the Arthropoda, including most notably the Crustacea, Arachnida, and Insecta. Such 

 a connection, nevertheless, is, in the author's opinion, obscurely traceable in the 

 direction of the Hypotricha, the interval separating the first-named highly 

 differentiated Metazoa from the Protozoic order being bridged over by the group of 

 the Wheel-animalcules or Rotifera. The passage from the Hypotrichous Cihata to 

 the Rotifera is indicated in two completely distinct directions. Firstly, in such a 

 type as the Dysteria armata of Huxley, PI. XLII. Figs. 27 to 30, in which the 

 complex buccal armature and jointed caudal style are so strongly suggestive of its 

 Rotiferan affinities that the animal was originally referred by ^Ir. P. H. Gosse to 

 the class in question. Secondly, in that aberrant group of the Rotifera, including 

 Ichthydium, Chcetono/tis, Turbandla, and a few others, recently incorporated together 

 by Metschnikoff and Claparfede under the title of the Gasterotricha,| which are all 

 distinguished by the absence of the customarily developed trochal discs and 

 complex mastax, while the entire ventral surface alone is clothed with fine vibratile 

 cilia after the manner of the most simple Hypotricha. It is at the same time 

 necessary to observe that among the generality of the Rotifera, both in their larval 

 and frequently in their adult state, the cilia form a single terminal wreath around 

 the so-called trochal disc, and consequently correspond closely with the Telo- 

 trochous larvre of the Echinoderms and Polychaetous Annelida previously described. f 

 In common with these lar\-je, they are likewise apparently phylogenetically derived 

 from the infusorial order of the Peritricha. In this connection it may be further 

 remarked that the two endoparasitic Peritrichous genera Ophryoscolcx and Eiito- 

 tfiiiiiim, distinguished by the possession of indurated carapaces and variously 

 modified spinous appendages, were originally referred to the Rotifera. In one of 

 these genera it is interesting to find that a second girdle of cilia is developed round 

 the centre of the body. The affinities of the Rotifera with the Crustacea are, as 

 indicated by Professor Huxley,§ possibly manifested in '■'■ Pedalion, with its jointed 

 setose appendages and curious likeness to some Nauplius conditions of the lower 

 Crustacea." It might be further added that many Rotifera, in common with 



• The so-called Atrochal larva of the Chaetopodous Annelid Serpula, as originally figured by 

 Stossich, and reproduced in F. M. Balfour's treatise on Comparative Embryology, vol. i., 1880, 

 conforms entirely to the Holotrichous plan of ciliation, and with its ventral oral, and postero- 

 terminal anal apertures, may be directly compared mth a Paramecium, or an embryo Turbellarian. 



t "Beitrage zur Kenntniss der Entwickelungsgeschichte der Chaetopoden," ' Zeit. Wiss. Zool.', 

 Bd. xix , 1869. 



X The resemblance between Rotifers and the lar\'ae of Echinoderms was pointed out by Professor 

 Huxley in his account of Lacinularia socialis in the ' Transactions ' of the Microscopical Society 

 for the year 1S51, and is further advocated in his ' Anatomy of the Invertebrata, p. 193, 1877. 



§ ' Anatomy of Invertebrated Animals,' p. 193, 1877. 



