60 AN ELEMENTARY TEXT-BOOK OF BIOLOGY. 



A large lateral cesopliageal vessel runs along each side of the 

 gullet, opening into the dorsal vessel in segment 10, and return- 

 ing blood from the gullet and pharynx. 



Course of the Circulation. The blood is kept circulating by 

 means of the hearts, which act as force pumps. Their walls are 

 muscular, and they contract rhythmically from above downwards, 

 the hinder ones contracting first. The large vessels also have 

 contractile walls by which the circulation is assisted. The blood 

 flows from back to front in the dorsal vessel, and the contrary 

 way in the ventral one. The parietal vessels return blood puri- 

 fied in the skin and nephridia to the dorsal vessels, which also 

 receives blood from the gut by means of the lateral cesopliageal 

 and efferent intestinal vessels. This blood passes forwards in the 

 dorsal vessel and through the hearts to the ventral vessels, in 

 which the blood flows backwards and passes by afferent trunks 

 to the gut, nephridia, and body-wall. 



The delicate networks of capillary vessels which complete the 

 closed blood-system are of great physiological importance, since 

 their extremely thin walls allow of diffusion. In this way, for 

 example, the tissues are nourished. 



There are no special organs of respiration and this function is 

 performed by the general surface of the body, the carbon dioxide 

 diffusing out of the blood-vessels underlying and penetrating the 

 skin, while oxygen diffuses into them. The red colour of the 

 blood is due to hemoglobin, a complex compound of carbon, 

 hydrogen, oxygen, nitrogen, sulphur, and iron, which enters into 

 loose combination with the oxygen taken into the body, readily 

 parting with it again to the tissues. 



(2) Coelom and Coelomic Fluid. The coslom or body-cavity is 

 imperfectly separated into compartments by the septa, and com- 

 municates with the exterior by the dorsal pores, excretory tubes 

 (nephridia), and oviducts. It contains a milky coagulable coelomic 

 (peri visceral) fluid, in which are suspended numerous irregular 

 nucleated cells (ccelomic corpuscles), with granular protoplasm, 

 capable of amoeboid movements. The body-cavity is lined by 

 coelomic epithelium composed of a single layer of flattened 

 nucleated cells, from which these corpuscles are constantly being 

 budded off. 



5. Excretory Organs. Each segment, with the exception of 

 the first three and the last, contains a pair of nephridia (segment*! 



