5(j THE BIOLOGY OF AN ANIMAL. 



2. Dorso-intestinal vessels, passing from the dorsal vessel 

 into the wall of the gut in the region of the stomach-intestine. 

 Of these vessels there are two or three pairs in each somite. 

 They are thickly covered (like the dorsal vessel in this region) 

 with pigmented " chloragogue- cells, " so that their red color is 

 usually not apparent. Unlike the aortic arches these vessels 

 break up on the wall of the intestine into capillaries which are 

 continuous with branches from the ventral vessel. 



3. Dorso-tegumentary vessels, passing from the dorsal vessel 

 along the dissepiment into the body-wall on each side. These 

 are small vessels that pass directly around the body to connect 

 with a longitudinal trunk (" sub-neural ") lying below the ven- 

 tral nerve-cord (see below), and giving off branches to the body- 

 wall, dissepiments, and nephridia. 



Course of the Blood. The precise course of the blood in 

 Zumbricus is still in dispute, though its more general features are 

 known. It is certain that the bulk of the blood passes forward in 

 the dorsal vessel, downward around the gut through the aortic arches 

 into the ventral vessel, and thence backwards towards the pos- 

 terior region. Its path thence into the dorsal vessel is doubtful. 

 The most probable view is that the blood proceeds from the ven- 

 tral vessel through ventro-intestinal vessels to the capillaries of 

 the intestine and thence to the dorsal vessel through the dorso- 

 intestinal vessels. It is possible, however, that the return path 

 is through the dorso-tegumentary vessels and that the dorso 

 intestinal carry blood from the dorsal vessel to the intestine. 



In the foregoing account only the more obvious features of the blood- 

 vessels have been mentioned, and many important details have been passed 

 over. The circular vessels of the stomach-intestine can be followed for 

 only a short distance out from the dorsal vessel, where they seem to break 

 up into a large number of small parallel vessels lying close together and 

 running around to the lower side. The efferent vessels do not directly join 

 the sub-intestinal, but empty into a sinus or vessel which runs parallel to 

 tne latter, closely imbedded in the wall of the stomach-intestine. The sub- 

 intestinal vessel proper is quite separate from the stomach-intestine, and 

 communicates by short branches (usually two in each somite) with the 

 vessel lying above it. This may be clearly seen in the region of the gizzard. 

 On this there is a variable number of small lateral vessels, which break up 

 partly into a branching network, and are partly resolved into extremely 

 fine parallel vessels surrounding the organ. On the crop are three or four 

 pairs of lateral branches from the dorsal vessel which branch out into a 



