BLOOD VESSELS OF LUMBRICUS, /Q 



capillaries of the body wall. This is perhaps an insuperable 

 objection to that theory of the circulation. This objection does 

 hold against the view of Perrier 1 and Benham, 2 according to 

 which the blood flows to the dorsal vessel in the parietals and 

 out from it in the dorso-intestinals. 



The study of the structure of the vessels shows that the move- 

 ment of the blood is determined by the structure of the walls 

 and by definite valves which several of the vessels possess. The 

 wall of the dorsal vessel consists (Fig. 2) of a lining endothelium 

 of very thin cells whose nuclei alone are usually visible ; a con- 

 nective tissue layer containing a few longitudinal (muscle?) fibers, 

 and a well-developed layer of circular muscle fibers. Outside 

 these are the chloragogue cells. To the layer of circular muscle 

 fibers are due the pulsations of the dorsal vessel, and thickenings 

 of this layer at certain points assist in the action of the valves, 

 as will be described below. The wall of the ventral vessel has 

 no circular muscle layer. Its lining endothelium is more con- 

 spicuous than that of the dorsal vessel and the connective tissue 

 layer is very thick. This is a strong fibrous layer and gives 

 great rigidity to the wall of the ventral vessel. Outside of the 

 connective tissue layer are a few (4 to 6) strands of longitudinal 

 fibers which take the same stain as the muscle fibers in the 

 sheath of the neighboring nerve cord. Outside these fibers is a 

 layer of peritoneum closely similar to that covering the inner 

 surface of the body wall. 



The subneural consists of only the endothelium and connective 

 tissue layer, outside of which is the sheath of the nerve cord. 

 This is the structure of the lateral neurals also, and of all the 

 smaller vessels. The dorso-intestinals and parietals present an 

 intermediate condition between those with and those without a 

 circular muscle layer. The dorso-intestinal vessels are devoid 

 of muscle fibers except at their dorsal ends where there is a thin 

 extension of the circular layer of the dorsal vessel for a short 

 distance. The parietals are provided with a thick band of circular 

 fibres close to their connections with the dorsal and the layer is 



1 Perrier, Edw., " Recherches pour servir a I'histoire des Lombriciens terrestres," 

 Nonv. Arch, du Mus. d' Hist. Nat., Paris, Tome 8, 1872. 



2 Benham, W. B., "The Nephridium of Lnmbricus and its Blood Supply," Q. J. 

 M. S., Vol. 32, p. 293, 1891. 



