180 OLIGOCHAETA 



The alimentary canal, which is ciliated throughout, may be divided into 

 a pharynx, an oesophagus, and an intestine ; the pharynx occupies the first segment 

 of the body; in the young worm, budded off from the parent, the pharynx appears 

 as an epiblastic involution; it is lined by cubical, ciliated cells, and has a layer 

 of circular fibres. In A. tenebrarum the hinder margin of the pharynx gives rise, 

 on either side, to a small, thick-walled diverticulum, to which the last of the muscular 

 bands supporting the pharynx are attached. Following upon the pharynx is 

 a narrow oesophagus, occupying two or three segments, and commonly furnished at 

 its commencement with an oval dilation. The intestine is wide to begin with, and 

 gradually narrows towards the terminally-placed anus. The structure of the intestine 

 and oesophagus is exceedingly simple ; the walls of the tube consist of a cubical, 

 ciliated epithelium, outside which is a peritoneal layer ; between the two, in the 

 intestine, is interpolated a vascular network or sinus (see p. 73), and the dorsal blood- 

 vessel lies underneath the peritoneal covering of the oesophagus. 



The vascular system is exceedingly simple. It consists merely of a dorsal and 

 ventral trunk, which are united in the anterior part of the body ; the dorsal vessel is 

 usually only recognizable in the oesophageal region, where it forms a pulsatory tube^ 

 not, however, lying freely in the body-cavity, but beneath the oesophageal peritoneum ; 

 anteriorly this vessel behind the brain divides into two trunks, which reunite to form 

 the ventral vessel ; in a few examples of A. hemprichii Vejdovsky observed a pair 

 of vessels given off from the cephalic ring, which, he believes, rejoin the dorsal 

 vessel. Behind the oesophagus there is no dorsal vessel ; it ends here in a network 

 of blood-vessels, ramifying in the walls of the intestine, beneath the peritoneal coat. 

 The ventral vessel, on the other hand, lies , perfectly free in the body-cavity; 

 anteriorly it is, as already mentioned, in communication with the dorsal-vessel by 

 a pair of lateral commissures passing round the pharynx. In the intestinal region it 

 gives off a number of trunks, sometimes regularly paired, sometimes not, which join the 

 intestinal network. This intestinal network appears not to exist in either A. quater- 

 nanum, A. variegatum, or A. headleyi; it is replaced in those species by a paired 

 blood-sinus^. Proper walls to the blood-vessels have not been clearly demonstrated. 

 Blood-corpuscles have, however, been found by Eichwald, Maggi ^, and Vejdovsky, 

 but only in the dorsal vessel. In several species there is a row of somewhat 

 fusiform cells in the dorsal vessel, which have yellowish fat-drops in their interior ; 

 I have suggested (4) that these cells probably represent a rudimentary dorsal organ 



1 I say 'paired blood-sinus' in deference to Vejdovsky; in A. headleyi (see my figure 68, PI. fig. 6) 

 the sinus did not seem to be paired, but to be continuous all round the intestine. 

 ' The corpuscles described by Magmji are, however, probably eoelomic. 



