718 PROCEEDINGS OF THE ACADEMY OF [DcC. , 



It is thus seen that the caudicle of Zygeupolia is a structure 

 from which many organs of the body are absent, namely: the ali- 

 mentary canal, the gonads, the rhynchocoel, the outer longitudinal 

 muscle layer and the cutis. 



The significance of the caudicle, however, is not clear, and 

 several explanations may be suggested : (1) The caudicle has 

 remained in a simple, primitive or embryonic condition, while the 

 rest of the body has become differentiated. 



(2) The caudicle is a degenerate structure, the degeneration of 

 certain organs having begun at the posterior end and continued 

 gradually forward. 



(3) The caudicle is a coenogeuetic structure, with a certain 

 physiological function. 



Of these views, no positive proof can be brought forward in 

 regard to the first two. The varying complexity of the caudicles 

 of different genera — for example, that of Zygeupolia and the caudi- 

 cle described by Burger (1895; for Cerebratulus marginatus, con- 

 taining all the organs of the posterior end of the body — might sup- 

 port the view of degeneracy, but this argument may hardly be 

 used until we know more of the origin and phylogeny of the cau- 

 dicle. 



It seems much more probable that the caudicle of the Nemer- 

 teans has arisen coenogenetically, and an explanation of its mode 

 of origin has been suggested by the comparison with a Rhabdocoel 

 Turbellarian, Macrostoma hystrix Oe., described by Graff (1882), 

 and figured on Taf. IV, Fig. 1. In this worm the posterior end is 

 expanded laterally and provided with abundant gland cells, making 

 an adhesive surface. Graff says, p. 240, the body is " hinten in 

 einen platten spatelformig erweiterten Schwanz ausgezogen. " It 

 would not be difficult to imagiue this posterior end becoming elon- 

 gated and more slender, until it is finally a thin filament. In a 

 similar way the Nomertcan caudicle may have arisen from a pos- 

 terior end, originally differentiated as au adhesive surface. The 

 observations of Johannes Muller (1854) and M'Intosh (1869) 

 show that at the present time the end of the caudicle frequently 

 acts as a sucker. 



M'Intosh (1869) believes that the central space in the caudicle 

 of Micrura purpurea is connected with the -circulatory system. 

 This, as stated above, is certainly true in Zygeupolia. The fact 



