The Eartkivorm. 25 



arrangement. The nephridia are tubes which, though very long, 

 do not occupy much space owing to the fact that each tube is 

 coiled several times upon itself in the way illustrated in the 

 figure (Fig. 11). The nephridium commences by an open 

 mouth, the ftmnel; this is immediately continued into a 

 delicate tube which passes backwards and passes through the 

 septum into the segment behind. It then winds about in the 

 way illustrated, and ultimately opens into a wider -sac, which 

 itself opens on to the exterior by the nephridiopore already 

 mentioned. 



The circulatory system is complicated and highly developed. 

 The main trunk is a dorsal vessel running upon the upper 

 surface of the alimentary tract. This is rhythmically con- 

 tractile, the contractions passing from behind forwards. This 

 communicates by several large, also contractile, "-hearts," 

 with the ventral vessel which is not contractile. This lies 

 above the nerve-cord ; the blood in it flows from before 

 backwards. Beneath the nerve-cord is another longitudinal 

 vessel, and in the oesophageal region a short vessel on 

 either side of the oesophagus which arises from the dorsal 

 vessel. The dorsal vessel in the segments lying behind this 

 lateral oesophageal vessel gives off a series of regularly 

 and segmentally arranged branches to the alimentary canal 

 and to the septa. Branches also arise from the ventral and 

 subneural vessels supplying the nephridia and body-wall. The 

 vascular system of the earthworm is everywhere a perfectly 

 closed system, consisting of tubes of regular diameter gradually 

 decreasing in calibre as the periphery is reached. The con- 

 tained blood is red in colour, the colour being due to hsemo- 

 globin dissolved in the plasma. In the plasma float a few 

 corpuscles, which seem to be little more than the nuclei of the 

 cells forming the blood-vessels. 



The earthworm has no special respiratory organs. The 

 blood capillaries approach so near the surface of the body — 

 actually into the epidermis in the clitellar region — that the 

 skin itself serves as a respiratory organ. 



The alimentary canal is a straight tube. The mouth, just 

 below the prostomium, leads into a buccal cavity, and that 



