ON CTENODRILUS PARVULUS. 593 



segment and vice versa. In all probability the bristles become 

 worn out in time and are then replaced by new ones. This 

 shows that we are not always justified in using the number of 

 bristles for diagnostic purposes. 



Ctenodrilus parvulus is covered by a very thin cuticula 

 and the under surface of the prostomium as well as the under 

 part of the mouth segment are ciliated as in Aelosoma. The 

 whole of the oesophageal cells and those of the intestine also 

 bear cilia. Under the lens the animal has a reddish appear- 

 ance, due to the bright red colour of the large stomach (fig. 1, 

 5^.),Avhichwill be dealt with later on. The oesophagus reaches 

 to the commencement of the third segment, the stomach 

 occupies generally three or four, and the remaining segments 

 contain the intestine. 



With regard to segmental organs there is only one pair of 

 them, viz. in the head segment (fig. 1, so.). In some of the 

 more transparent specimens they are readily seen, but the 

 study of their histological structure offers great difficulty. 



The vascular system, consisting of a dorsal {d. v.) and a 

 ventral vessel {v. v.), is not always distinctly visible. The 

 blood is colourless and does not contain any blood-corpus- 

 cles. There is a dorsal vessel, existing only in the three 

 anterior segments. It takes its origin at the junction of 

 the oesophagus and stomach and its lumen is gradually 

 narrowed down as it reaches the head segment. In the 

 first segment it divides into two branches, which surround 

 the oesophagus, and, joining again below into one, forming the 

 ventral vessel. 



It now remains to say something about the movements of 

 the animal. When isolated it quickly draws itself together and 

 remains in a curved position for some time. It then protrudes 

 the large bifid lower lip, which is no doubt used as a " point 

 d'appui,^^ in order to draw the body forward. But to do this 

 the longitudinal muscles are gradually shortened, causing the 

 body to swell up. The swelling is first seen near the hinder 

 end, but soon travels towards the head, which is thus moved 

 onward. Having hereby changed its position, before it ventures 



