812 PROFESSOR J. STEPHENSON ON 



the side of the anus in Potamilla. Capitella frequently, the Eriographidse [Myxicola 

 dinar densis, M. infundihulum), and Sabellidse {Potamilla) regularly, move with the 

 posterior end foremost ; and this manner of locomotion, along with the evidently highly 

 developed sensibility and the continual exploring movements of the anal extremity, 

 indeed the whole behaviour of these worms [Myxicola and Potamilla), makes it im- 

 possible to resist the belief that the physiological head of the animal is the morphological 

 posterior end of the body. 



(4) I have already mentioned that my object in studying the Polychseta was to 

 extend the observational basis on which the views, previously put forward, on the origin 

 and mode of evolution of the vascular system in the Annelida were founded. The 

 results of my investigations are evidently in harmony with those views, and in a general 

 way confirmatory. Since, however, they add nothing of importance to the argument, 

 it seemed better to develop the discussion on the basis of the Oligochseta, and to give 

 the results of the Polychsete investigation separately, as has been done in the present Part. 



The reasons why the material gathered from the lower families of the Oligochseta is 

 better adapted to serve the purpose of a discussion on the evolution of the vascular 

 system seem to be the following : — In the first place, the Oligochseta are distributed into 

 far fewer families than the Polychseta ; the anatomical material which falls to be con- 

 sidered is therefore more easily mastered, and, when mastered, is more easily arranged. 

 Secondly, the Microdrili are for the most part small transparent worms which can be 

 observed microscopically without difficulty ; while the Polychseta are often large and 

 opaque, and hence the points in question — ascending ciliary action, antiperistalsis, the 

 relation of intestinal to vascular contractions — are often impossible of discovery by 

 ordinary means. Thirdly, the aquatic Oligochseta appear to contain a larger number of 

 the lower stages of the evolutionary series ; indeed, it may be said that they represent 

 a generally more primitive level than any of the Polychseta. 



Since, therefore, it appears that the observations on the Polychseta fit into the 

 scheme of evolution of the vascular system as already outlined, and that thus there is 

 nothing that contradicts, and much that agrees with, the results arrived at from a study 

 of the Oligochseta, it does not seem too much to say that the views previously put 

 forward receive a useful confirmation from this series of observations. 



A few parallels with the Oligochseta may be noticed. 



(a) In the Serpulimorpha (Sabellidse, Eriographidse, Serpulidse) there is a perienteric 

 sinus within the muscular coat of the intestine, but no differentiated dorsal vessel behind 

 the oesophagus ; the ventral vessel, however, exists throughout the length of the body 

 (I take these anatomical data from Fuchs). Here the antiperistaltic contractions of the 

 muscular coat drive forwards the blood in the sinus ; they have usually, however, com- 

 paratively little effect in narrowing the lumen of the gut. The anatomical and physio- 

 logical conditions are comparable to what is found in the Enchytrseidse, and represent 

 one of the lower stages in the evolution of the vascular system, one in which the 

 vascular is, in great part of its extent, not emancipated either anatomically or 



