340 ZOOLOGY. 



There is some evidence that the intestinal epithelium 

 actually ingests food-particles : if so the process of digestion 

 must in part be infra-cellular, as in Hydra. 



5. Vascular System, The ccelomic fluid, as in 

 Amphioxus, probably carries on part of the functions of 

 blood ; but there is in addition a true vascular system, with 

 a red blood circulating in definite vessels. As in the 

 Vertebrata, the red colour of the blood is due to haemo- 

 globin, but here it is dissolved in the plasma and not in 

 special corpuscles. There are small corpuscles in the blood, 

 but they are colourless. The chief vessels are longitudinal, 

 a dorsal one (fig. 176) above the alimentary canal, and four 

 ventral ones below it, whose relation to the nerve- cord is ex- 

 pressed in their names supra-neural, lateral-neural (paired), 

 and infra-neural. In each somite from 6 to 1 1 there is a 

 pair of contractile vessels, connecting the dorsal and supra- 

 neural vessels around the oesophagus : these are commonly 

 called the " hearts " (fig, 176), there being no better claimant 

 to that title in the earthworm. In each somite there is a 

 connecting vessel between the dorsal and infra-neural, 

 running in the body-wall, and there are branches to all 

 the tissues from one or other of the main vessels. 



In rough outline, there is a curious similarity between 

 the circulation in the earthworm and that in Amphioxus. 

 In both, the main vessels (indistinguishable as veins or 

 arteries) run along the dorsal and ventral sides of the 

 alimentary canal, and are connected towards the anterior 

 end by a paired series of contractile vessels embracing that 

 canal. But in Amphioxus the direction of circulation is 

 forwards in the ventral vessel, up through the contractile 

 vessels, and backwards in the dorsal vessel. In the earth- 

 worm the course of the blood is just the reverse. Moreover, 

 the connecting vessels are not associated with gill-slits. 

 Indeed, there are no special respiratory organs the 

 epidermis and cuticle are sufficiently thin to allow of 

 diffusion of gases between the air and the blood in the 

 sub-epidermal capillaries. There is no lymphatic system, 

 but the coelomic fluid certainly carries on the functions of 

 lymph, and in some degree those of blood also. 



