INTESTINAL RESPIRATION IN ANNELIDS. 777 



Lang sets forth his conclusions in the form of a number of theses (pp. 192-200) ; 

 of these, those which more particularly agree with the views put forward above are 

 the following : — 



(1) (Theses 2, 3). — Though, as will be seen below, the matter of these two proposi- 

 tions is not completely in accord with my views, one idea contained therein finds 

 expression also in the present paper : viz. that the blood-vascular system originates 

 in the diffusion of fluid from within the intestinal cavity into a space (or series of 

 spaces) outside the epithelial layer. 



(2) (Thesis 11). — Here is expressed the view that the first vessel which separated 

 from the perienteric sinus, and so became independent, was the ventral vessel ; and 

 that thus the return of the blood, which travels forwards in the sinus, was provided for. 



(3) (Thesis 28). — The musculature of the longitudinal vessels is here derived from 

 the musculature of the intestinal wall ; at first, before the complete separation of these 

 vessels from the sinus (I should prefer the word "plexus" where the reference is to the 

 primitive condition), the muscular coat would be wanting above (in the case of the 

 ventral) or below (in the case of the dorsal vessel) ; a complete investment would be 

 attained when the vessels had become fully separated. This agrees with the physiological 

 observations which I have recorded above, and describes the manner in which the 

 anatomical difl"erentiation, which corresponds to the physiological differentiation of two 

 separate contractile rhythms, must have taken place. 



If in what follows I dwell at greater length on certain of the difierences between 

 Lang's conception of the early history of the vascular system and my own, it will 

 be understood that this is not because the importance of the differences outweighs the 

 essential agreement, but because of the necessity I am under of justifying, to the extent 

 of my ability, my temerity in opposing myself to so distinguished an author and 

 investigator. 



(1) (Theses 2, 3).— Lang, deriving the earliest Annelids from Turbellarian-like 

 forms with metamerically branching alimentary space and numerous gonadial sacs 

 occupying the intervals between the neighbouring branches, supposes that these 

 sacs swelled to form a chambered coelom ; that in proportion as this took place the 

 interposed branches of the alimentary cavity shortened and disappeared, leaving a 

 space in their stead ; which, becoming filled with nutritive fluid diffusing through from 

 the intestinal cavity, formed the origin of the vascular system. At first this con- 

 sisted of a (continuous, vide Lang's fig. 2) peri-intestinal sinus with, as prolongations 

 therefrom, septal and mesenterial sinuses between adjacent chambers of the coelom. 



On this 1 may perhaps remark, that if the branches of the alimentary cavity shrank 

 only in proportion as the gonadial sacs swelled (in dem Masse, als sich die Sackgonaden 

 zu Gonocolsacken erweiterten), no space would be left. If such were left, it would not 

 represent a peri-intestinal sinus (since nothing is said as to the shrinking of the main 

 intestinal tube), but a series of septal sinuses. And lastly, my main point is that the 

 transudation of fluid, when the latter first began to accumulate between intestinal 



