776 PROFESSOR J. STEPHENSON ON 



passed, after being reduced to solution, through the epithelial wall ; similarly a certain 

 amount of water carrying oxygen and dissolved matters, introduced at the anus. 

 These fluids tended to accumulate in a series of spaces which they made for themselves 

 external to the alimentary epithelium ; as the spaces increased in size, they became 

 confluent ; and thus was established the alimentary network, from which by progressive 

 differentiation the vascular system was later evolved. 



4. The Theories of Lang and Vejdovsky. 



The foregoing considerations are based on observations on one order only — that of 

 the Oligochseta, and of certain families only within that order. Though the.se con- 

 siderations apparently help to explain the origin of the vascular system in the group 

 on which the observations have been made, it is quite possible that the explanation 

 may not be of universal application ; it will, however, in all likelihood be found applic- 

 able to the Annelida in general, and consequently to any groups which can be shown 

 to be descended from these. But it appears possible that the vascular system is not 

 homologous throughout the animal kingdom, and that in the Nemertines, for example, 

 it may own a diS'erent origin. 



The observations also, while partly anatomical, are largely from the side of 

 physiology. I have not, however, attempted to emulate the painstaking histological 

 researches which have of late been directed, by a number of observers, towards the 

 elucidation of the problem here considered. But such agreements as are found between 

 the results here attained, and those reached by other workers, on other groups and 

 along other lines, will be of value from the very fact that the path here pursued has 

 been a diftereut one. 



The first agreement that I have in mind is that which exists between the results 

 here reached and those put forward by Lang (29), in respect of the same question. 

 Lang's theory, based on anatomical and also especially on histological evidence, 

 collected in great quantity and detail, and discussed in the work just referred to, 

 supposes the vascular system to have had its origin in the diflfusion of fluid into the 

 space between the epithelium of the alimentary tube and the surrounding mesoblastic 

 elements, which, in accordance with the gonocoel theory of the origin of the cceloni. are 

 conceived as genital pouches. At its first appearance the system is thus constituted 

 by a perienteric sinus, with ring-like centrifugal extensions between the transversely 

 placed walls of successive gonadial chambers, and dorsal and ventral extensions in the 

 sagittal plane (mesenterial sinuses) between the adjacent inner walls of the right and 

 left chambers of the same pairs. The main blood-vessels, dorsal, ventral and lateral 

 commissural, are specialisations of this common reservoir, determined as to their position 

 by tlie places in which the neiglibouring walls of the coelomic (gonadial) chambers come 

 together and fuse. The similarity, in a general way, of the view put forward in the last 

 section with the central idea of Lang's theory needs no emphasis. 



