84 J. B. JOHNSTON. 



and project so far into the ventral vessel that they might be mis- 

 taken for valves proper to the ventral vessel itself. 



The course of the blood flow is determined by the disposition 

 of the valves as well as by the direction of the pulsations, and 

 there is evidently entire agreement between the results of the 

 physiological experiments and anatomical investigation. It is 

 obvious that in small vessels or in such as receive blood from a 

 capillary system so that there is no great pressure in the usual 

 course, there may occur temporary reversals of flow due to 

 movements of the body or other causes. Such reversals might 

 most readily take place in the subneural vessel and such phe- 

 nomena are probably the basis for Harrington's statement that 

 the blood flows now forward, now backward in the subneural. 

 However, the general course of the blood flow is strictly deter- 

 mined, as shown by the consistent experimental and anatomical 

 results, and no considerable or long-continued reversal or inter- 

 ruption of the usual current are possible except as the result of 

 violent interference such as decapitation of the worm. 



The valves in the vessels have received very meager notices 

 heretofore. The mention of valves in the dorsal vessel by Ben- 

 ham has been noticed above. A recent writer 1 has mentioned 

 the presence within the dorsal vessel of cells similar to the chlora- 

 gogue cells. These are also doubtless the valves of the dorsal 

 vessel. 



EXPLANATION OF FIGURES. 



Abbreviations : b.w., body wall ; c.t , connective tissue layer of blood vessels ; d., 

 dorsal vessel ; d-i., dorse-intestinal vessel ; d-t., dorso-typhlosolar vessel ; end. , endo- 

 thelial lining of vessels ; i.v.p., vascular plexus of intestine ; /.., lateral neural ves- 

 sel ; ;., layer of circular muscle fibers in walls of vessels; ;//>//., nephridium ; /., 

 parietal vessel ; s., septum ; s-n., subneural vessel ; t.s., typhlosolar sinus ; ?'. , ven- 

 tral vessel; va., valve; i>.i., ventro-intestinal vessel. 



'Rice, Win. J., "Studies in Earthworm Chloragogue," BIOL. BULL., Vol. 

 III., Nos. 1-2, 1902. 



