268 W. F. E, WBLDON. 



of a single ciliated ring in an undoubted Rhabdocoel. The 

 assumption of a pelagic life might easily cause in any Rhab- 

 docoel a hypertrophy of the cilia in certain definite regions and 

 the consequent appearance of ciliated bands; and it seems safe 

 to predict that a more thorough investigation of the pelagic 

 inhabitants of those warm seas which are most favorable to 

 the development of surface faunas will reveal the existence of 

 genera in which this character has been developed. 



The researches of Lang on Oligocladus and Cyclo- 

 porus^ have shown that at least in Polyclads there is no diffi- 

 culty in the temporary establishment of an anus in any region 

 of the body, and when this is once recognised the passage from 

 a temporary to a permanent condition is easy. 



The pharynx of Dinophilus and of the lower Chsetopods 

 offers another strong proof of Turbellarian affinities. On com- 

 paring the diagrams given in figs. 11 to 16 we see that the 

 stomodseum of DinophiluSj Polygordius, and Histriob- 

 della possesses a posterior muscular thickening lying in the 

 wall of a lateral outgrowth from the pharynx, which is in all 

 cases conceivably, and in Dinophilus certainly, eversible. In 

 the embryo Terebella (fig. 14) a similar posterior outgrowth 

 from the stomodseum exists, which subsequently^ envelopes 

 the whole circumference of the pharynx, and constitutes the 

 rudiment of the pharyngeal armature. In Nais (fig. 15) we 

 have a similar muscular thickening on the anterior wall of 

 the stomodseum. 



These facts receive at least a plausible explanation, if we 

 suppose that the various forms of pharyngeal apparatus just 

 mentioned are derived from a structure which primitively sur- 

 rounded the whole organ, persistence in the posterior region 

 only being in such forms as Polygordius, perhaps associated 

 with the filling up of the prse-oral lobe by the brain, while the 

 existence of an elongated proboscidiform prostomium in Nais 

 renders it most convenient to preserve the musculature in 

 front. But such a circumoesophageal apparatus as is here in- 



' Lang, op. cit., pp. 155, et seq. 



' Salensky, ' A.rchives de Biologie,' t. iv. 



