498 ANNELIDA. 



In the lower Oligochaeta the walls of even the large vessels 

 consist merely of a thin, apparently structureless, membrane, with 

 here and there a nucleus. In the contractile vessels the contractility 

 would appear to reside in this membrane. In the higher forms the 

 large vessels possess muscular fibres in their walls, and an epitheloid 

 lining. 



The coelom is spacious, and is always divided by transverse 

 segmental septa into chambers, which communicate usually above 

 the nerve-cord. It contains a colourless fluid with amoeboid cells, 

 and communicates with the exterior through three sets of aper- 

 tures the nephridia, the generative ducts, and the dorsal pores. 

 The dorsal pores are present, one in each segment, in many Terricolae, 

 but in the Limicolae are confined to one, in the prestomium. They 

 are usually placed intersegmentally over the septa. Their function 

 is unknown, but in some worms fluid has been seen to exude 

 from them. 



The lining of the coelom consists mainly of a flat epithelium, but 

 on some parts of the alimentary canal it has the form of large, yellow 

 cells the so-called yellow or chloragogen cells which are loaded 

 with pigment grains. These cells are probably excretory in function, 

 and allow their contents to escape into the body-cavity, and to pass 

 thence to the exterior through the nephridia. The coelom is sub- 

 divided, as stated above, by the transverse septa, and in certain forms 

 (Libyodrilus, Bmnchiura) there are obliquely placed sheets shutting 

 off a lateral, parapodial-like section of it, as in some Polychaets. 

 Further, portions of the coelom are sometimes shut off from the 

 rest in connection with the generative organs (see below). There 

 is no longitudinal dorso-ventral mesentery (except in Criodrilus), but 

 traces of the ventral mesentery are sometimes left. 



Sometimes certain of the larger blood vessels are enclosed in 

 perihaemal spaces. Whether or not these are coelomic is unknown. 



Nephridia are absent from a certain number of the anterior 

 segments ; and in the Limicolae the generative segments are without 

 them in the breeding season. They are of two kinds: (1) the 

 meganephric, or ordinary kind, such as is found in the Limicolae 

 and in many of the Terricolae, and (2) the pledonepliric or diffuse 

 variety, found in some members of the families Perichaetidae, 

 Cryptodrilidae, and Acanthodrilidae. The two kinds may co-exist 

 in the same animal, and even in the same segment (Megascolides). 



In Lumbricus we find the meganephric condition. The nephridia, 

 of which there is a single pair in each segment, consist of a long 



