THE ANATOMY. VASCULAR SYSTEM 65 



varies in the diflFerent groups ; they are more complex in the larger forms and on 

 the whole less so in the smaller worms. 



§ I. Histology of blood-vessels. 



The blood-vessels of the Oligochaeta are tubular, rarely lacunar ; the larger 

 vessels, some of which are contractile, have thicker or thinner walls, and give rise 

 in the higher Oligochaeta to an extensive system of capillaries. In the larger 

 vessels circular as well as longitudinal muscular fibres exist and the vessels are 

 .lined by an epithelium and covered by the cells of the peritoneal investment. 

 There is an increase in the elaboration of structure of the principal (dorsal) 

 blood trunk, as we pass from the lower to the higher Oligochaeta. In Aeolosoma 

 the dorsal vessel consists^ according to Vejdovsky, of no more than an exceedingly 

 fine membrane, in which no structure, muscular or other, could be detected ; it is 

 covered of course by the peritoneum. Very little more can be said of the Naids and 

 Enchytraeidae ; there is simply a delicate membrane covered with peritoneum. In 

 Ghaetogaster, however, which I reckon a Naid, it is possible to recognize faint longi- 

 tudinal as well as transverse striae. Something similar is figured by Taubee (3, Tab. iii, 

 fig. ii) for Rais elinguis, and Vejdovsky figures (24, Tab. iv, figs. 20, 21) and describes 

 contractile cells (besides the peritoneal cells, with which they must not be confused) 

 as constituting the walls of the dorsal vessel of Stylaria lacustris. One is inclined 

 to suppose that the dorsal vessel of Aeolosoma, since it is contractile, will prove to 

 possess muscular fibres. In the forms that have been hitherto considered there seems 

 to be no definite lining of epithelium to the dorsal vessel. In the higher Oligochaeta, 

 on the other hand, the epithelial lining consists often of very large and conspicuous 

 cells. In Phreoryctes, for example, I have figured (18) a transverse and longitudinal 

 section through one of the perioesophageal vessels (which have the same structure as 

 the dorsal vessel) which show (Figs. 8, 9 of plate) the cubical cells lining the lumen 

 of the vessel ; the strong circular and longitudinal muscular layers of the vessel are 

 also shown. Vbjdovsky's figures (24, Taf. xiv, figs. 10-13) show the lining epithelium 

 and at any rate a circular layer of muscles in Griodrilus lacuum; while Pbrriee 

 has recorded a similar structure for Urochaeta. Naturally the circular ^ layer is much 

 more important than the longitudinal,, and it has therefore been often the only one 

 figured. The blood itself is usually red; yellower in Naids, and colourless in 

 Aeolosoma^. The colour, as was first shown by Lankestek, is due to haemoglobin. 



' Both circular and longitudinal fibres are described in the text (p. ii8). 

 " Lankester says that it is pink in Ae. quatemarium. 



K 



