CLASS II. CILIATA. 



condition by the possession of two, and, preceding the act of fission, four, ciliary girdles. 

 An approximate estimate of this resemblance may be gained by a comparison of the 

 figures just cited with the accompanying woodcut illustration, Fig. 9, of the earliest 

 larval or Echinopaedium phase of a Feather-star, Comatula, reproduced from 

 Haeckel's Natiirliche Schopfungs-Geschichte.' The Echinus, the Star-fish, or the 

 Holothurian, one and all, present an identical or but slightly modified initial structure, 

 and may therefore be consistently regarded as the descendants of a similar simple 

 Peritrichous archetype. 



12 



The figures, bracketed in pairs, represent six larval Metazoa, with their respective Infusorial isomorphs, as cor- 

 related in the accompanying text. 



Fig. i, an Astomatous larval Coslenterate or Planula; Fig. 2, an Astomatous Holotrichous Infusorium, Opalina; 

 _'. 3, a larval Aproctous Turbellarian ; Fig. 4, a Stomatode Holotrichous Infusorium, Paramizciunt ; Fig. 5, a larval 

 Nemertian, Cephalotkrix, after Macintosh ; Fig. 6, a Cilio-Flagellate Infusorium, Melodinium ; Fig. 7, a Telptro- 



chous Annelid larva, after Gegenbaur; Fig. 8, a Peritrichous Infusorium, Telotrochidium ; Fig. 9, a Mesotrochous 

 Echinoderm larva, after Haeckel ; Fig. 10, a Peritrichous Infusorium, Didinium (prior to subdivision) ; Fig. n, a 

 typical larval Polyzoon, after Barrois ; Fig. 12, a Peritrichous Infusorium, Vorticella. 

 The letters or and an indicate respectively the oral and anal apertures. 



The small group of the Gephyrea, including notably Sipunculus and Priapulus, 

 frequently cited as possessing structural modifications that unite the two groups 

 of the Echinodermata and Annelida, is of interest in connection with the present 

 subject of discussion since the embryonic zooids, in at least Sipunculus, accord 

 in their Peritrichous plan of ciliation with those of both the first-named group and 

 the Polychaetous section of the Annelida. The ciliated embryos of the Polychaetous 

 Annelida, while thus conforming to the Peritrichous type, are found within such 

 limits to exhibit three somewhat diverse modifications. As explained by Professor 

 Huxley in his ' Anatomy of the Invertebrata,' p. 243, the cilia in some cases form 

 a broad zone which encircles the body, leaving at each end an area which is either 

 devoid of cilia, or, as is frequently the case, has a tuft of cilia at the cephalic end. 

 Such larvae are termed " Atrocha." In other embryos the cilia are arranged in one 

 or more narrow bands which surround the body. Where two bands of cilia are 

 developed, the one encircling the body immediately in front of the mouth, and the 

 other around the anal end of the embryo, the larvae are called " Telotrocha." In 

 the third modification one or many bands of cilia surround the middle of the body 

 between the mouth and the hinder extremity, such larvae being distinguished by the 

 title of " Mesotrocha " ; a supplementary tuft of cilia also, in the case of the 

 Telotrocha, being not unfrequently attached to the centre of the praestomium or pre- 

 oral region. While such Peritrichous Infusoria as Urocentrum and Didinium conform 

 more essentially to the Telotrochal larval type, in the intermediate structural form 

 Calceolus (Peridinium) cypripedium of H. James-Clark, PI. XXXII. Figs. 23 and 24, 

 having, with the exception of a bare cap-like anterior region, the whole surface of 

 the body clothed with vibratile cilia, a decided approach is made towards the 



