OLIGOCHAETA. 497 



tubes, run along the dorsal side of the conjoined nerve cords in the 

 neurilemma. They are called the giant fibres, and are probably large 

 medullated nerve fibres. A number of visceral nerves are given off 

 })j the circumpharyngeal commissures to the pharynx and constitute 

 the visceral system. 



Eye-spots are present in a few of the aquatic forms, but they are 

 absent in the terrestrial forms ; in spite of this fact the earthworms 

 appear to be able to distinguish light from darkness. There appears 

 to be no auditory sense. 



The alimentary canal is often divided into several regions, the rela- 

 tions of which are most complicated in the Terricolae. In Lumbricus 

 the buccal cavity leads into a muscular pharynx, which is probably 

 used for sucking. This is followed by a long oesophagus extending 

 into about the fourteenth segment, and furnished with a thick layer of 

 glandular cells, and several dilated glandular appendages, the calcareous 

 ijlands. The oesophagus is dilated behind into a crop which leads into 

 a muscular gizzard. The gizzard (double in Moniligastridae and Ben- 

 hamia) has muscular walls and a chitinous lining, and it often contains 

 small stones; it leads into the straight intestine, the dorsal wall of 

 which is pushed inwards so as to form a longitudinal fold, the 

 typlilosole (comparable to a spiral valve). The intestine is dilated in 

 each segment and opens at the hind end by a terminal anus. In 

 the Limicolae the alimentary canal is simpler by the absence of a 

 gizzard ; a pharynx and oesophagus however are always present. 



The vascular system is a closed system of tubes containing a red 

 fluid, in which colourless corpuscles float. The red (yellow in thin 

 layers) colour is due to haemoglobin in solution in the plasma. 

 Although in solution, the haemoglobin does not appear to diffuse 

 through into the tissues or body spaces, the fluids of which are 

 colourless. The principal blood vessels are (1) a longitudinal dorsal 

 vessel, contractile from behind forwards ; (2) a ventral subintestinal 

 vessel also longitudinal, but noncontractile, and sometimes (3) a 

 subneural longitudinal vessel beneath the nerve-cord. These longi- 

 tudinal vessels give off lateral branches all along their course, which 

 themselves branch in the skin, nephridia, and other organs. In 

 some of the anterior segments there is a single pair of dilated lateral 

 vessels, which pass round the gut and open into the subintestinal 

 vessel : these are the so-called hearts, and are contractile from above 

 downwards. The dorsal vessel is sometimes double, either wholly or 

 in part. In some forms there appears to be a perienteric blood-sinus 

 in the intestinal wall, from which the dorsal vessel arises. 



2 K 



