356 OLIGOCHAETA 



the other organs of the body. The main trunks are a dorsal 

 and a ventral longitudinal, which communicate directly in the 

 anterior end of the body by large transverse contractile trunks, 

 the so-called hearts (see Fig. 190, 6). The dorsal vessel is also 

 contractile, but not the ventral, or, when it occurs, the sub- 

 nervian. The vascular system has many degrees of complexity 

 in different families ; it is simpler in the smaller aquatic forms. 

 The blood is usually red, and the pigment which is suspended in 

 the plasma is haemoglobin. The blood is corpusculated. 



Excretory Organs. There appears to be a great deal more 

 variation in the structure of the excretory system than there is 

 in many other groups. For a long time only Lumbricus and a 

 few of the aquatic genera were known as regards their excretory 

 systems. In these there is a pair of excretory organs or 

 nephridia in nearly all the segments. These are much coiled 

 tubes, in which it is always possible to recognise three divisions. 

 The nephridium commences with an orifice of a funnel-like 

 character, fringed with long cilia, and opening into the body- 

 cavity ; from this springs a tube, which immediately perforates 

 the septum lying between the segment which contains the 

 funnel and the following one ; this tube has the peculiarity 

 first pointed out by Claparede of being excavated in the sub- 

 stance of cells ; the glandular part of the nephridium is a row 

 of cells which are bored through by a continuous canal, the 

 walls of which are here and there furnished with cilia. It 

 often happens that the main canal gives off minute lateral rami- 

 fications, which may even form a kind of network round the 

 principal canal. The terminal section of the nephridium is a 

 muscular sac which opens on to the exterior by a pore, and from 

 which the products of excretion are from time to time evacuated 

 by contractions of its walls. This is a brief statement of the 

 main facts in the structure of those Oligochaeta in which there 

 is a single pair gf nephridia to each segment of the body ; small 

 differences of more or less importance occur. In Chaetogaster, for 

 example, there is no trace of a funnel ; in some genera the 

 terminal sac is much reduced or unusually extended, being even 

 sometimes provided with a caecum of moderate dimensions. In 

 Acanthodrilus novae-zelandiae and a few other species the point 

 of opening of the nephridia varies from segment to segment, 

 though it always bears some relation to the chaetae. In these 



