82 OLIGOCHAETA 



by the extension of the blood capiUaries to the epidermis itself; this interesting fact 



was first discovered by Lankestee. I myself extended this discovery (3) to 



Earthworms, and in the genera Megascolex, Perichaeta, Criodnlus, Moniligaster, &c., 



as I and others have shown, the epidermis of the body in general— not merely the 



clitellum— is vascular. As a general rule the lower Oligochaeta, in which the 



dermo-muscular tube, in correspondence with their small size, is actually— sometimes also 



relatively— thinner, than in the terrestrial worms, have no development of a capillary 



system in the skin. This rule, however, is not without exceptions. LimnodrUus 



Hoffmeisteri has tufts of blood capillaries which arise from the peri-intestinal trunks 



and enter the body wall, reaching even to the epidermis itself [Vejdovsky, 24, Taf. 



viii, figs. i6, 17]. More highly developed still is the integumental vascular network 



of Ilyodrilus (Stolc 3) and Branchiura (Beddaed 58). Many Tubificidae and Lum- 



briculidae, appear to respire chiefly at the posterior end of the body ; they live in the 



mud at the bottom of ponds, ditches, &c., with the head implanted and the tail 



waving about in the water. Now in the worms which have this habit, and for the 



matter of that in others too, the body gradually diminishes in calibre and in the 



thickness of its walls towards the anus ; hence the blood is necessarily brought into 



closer relation with the surrounding water. There is in these Oligochaeta, at least, 



a commencing localization of the respiratory function to the tail end, in accordance 



with the attenuation of the body in this region. And we find that when branchiae 



are developed they are in every instance but one confined to the caudal extremity. 



For a long time the only Oligoehaet known, which could be said to possess special 

 respiratory organs was Dero. This genus of Naidae is remarkable for possessing at 

 the end of the body an expanded hood formed by the widening out of the anus; 

 from the inner surface of this arise two pairs of (usually) cylindrical processes. 

 These processes, as well as the hood which covers them, are ciliated; they can only 

 be moved by means of intrinsic muscular fibres. The principal descriptions and 

 illustrations of this branchial area are those by Peebieii (10), and Bousfield (3)- 

 In describing the organ I shall follow Peebier's description and illustrations of 

 Bero ' ohtusa ' ( = really B. perrieri, Bousf.). The branchial processes themselves are 

 covered with a ciliated epithelium and their cavities are largely occupied by spindle- 

 shaped or stellate muscular fibres ^- The vascular supply is derived from a direct 

 continuation of the dorsal and ventral vessels. The ventral vessel passes undivided 

 to about the middle of the hood; here it divides into two trunks which run round 

 the margin of the hood and give ofi" six branches — one to each of the branchial 



' In D. digilata there are also retractor muscles for each braiichia. 



