144 ORGANS AND ORGAN SYSTEMS 



reappear in the absorptive cells; chlorophyll and fat have also 

 been detected. 



The Rectum- -The rectum of the earthworm is the posterior 

 part of the intestine, which bears no typhlosole. It opens to the 

 outside through the anus which is provided with a sphincter 

 muscle. According to Hensen, each worm deposits 1/2 gram 

 of faeces in 24 hours, and these faeces make up the castings 

 shown by Darwin to have great economic importance. 



Further Fate of the Absorbed Nutriment. The gut walls of the 

 earthworm are richly supplied with blood vessels and finer 

 capillaries which form a vascular network throughout the entire 

 intestine. It is highly probable, although never demonstrated, 

 that the absorbed food material is passed directly into the blood 

 through the walls of these vessels. 



E. BLOOD VASCULAR SYSTEM. In the earthworm it is the 

 plasm or fluid that is colored red by haemoglobin, while the 

 living cells of the circulation are colorless. As in vertebrates, 

 the blood flow is continuous, the circulation being closed 

 throughout, i.e., the blood is always in tubular vessels. The 

 main trunks are (i) a dorsal vessel lying on the digestive tract 

 and giving off in the anterior segments five pairs of enlarged 

 vessels, the so-called "aortic arches' 1 or loops. These loops 

 are connected below the oesophagus with (2) a ventral vessel 

 also running the entire length of the animal below the digestive 

 tract. Both dorsal and ventral vessels give off branches to the 

 adjacent tissues. One pair (parietals) run from the dorsal 

 vessel in each somite and along the dissepiment to the body wall, 

 where they split into numerous fine branches (capillaries), pene- 

 trating the dermal musculature and the epithelium. Other 

 branches of the dorsal vessel are given off to the digestive tract, 

 ending in capillaries in the walls of the stomach-intestine and 

 other organs. In the anterior region two lateral vessels are 

 given off, which supply the reproductive organs. Three other 

 longitudinal ventral vessels run the length of the worm; one, 

 the sub-neural vessel, lies below the nerve cord, while two 

 others, lateral-neurals, are embedded in the connective tissue 

 about the nerve cord, one on each side (Figs. 55 and 57). 



