No. 2.] STRUCTURE OF BIMASTOS PALUSTRIS. 489 



find even such rudimentary organs as Beddard (i) has described 

 in Allurus. 



Vascular system. — The vascular system has not been in- 

 vestigated, except in a very general way. There are three 

 longitudinal trunks running from end to end, or nearly so; 

 these are dorsal, sub-intestinal, and sub-neural. 



The dorsal trunk is single throughout, and in the anterior 

 region gives off five pairs of somewhat moniliform contractile 

 vessels in somites VII, VIII, IX, X, and XI. The first four 

 of these always flow directly into the sub-intestinal trunk ; the 

 fifth pair sometimes first pass into the intestinal wall, from 

 which they emerge laterally and thence flow into the sub- 

 intestinal trunk. In the latter case the free portion of the 

 vessel is only about one-third as long, and is much weaker than 

 its fellows. In front of these lateral arches the dorsal vessel 

 ramifies in the buccal and pharyngeal mass. 



A pair of oesophageal vessels arise from the sub-intestinal 

 trunk in somite X and pass laterally forward to supply the 

 tissue surrounding the anterior portion of the alimentary 

 canal. 



The sub-neural vessel lies within the neural sheath, and 

 gives off branches, along with the nerves, to supply the body 

 walls. 



A vessel in frequent communication with the dorsal trunk 

 traverses the typhlasole. 



Cejitral nervous system. — The main features of the nervous 

 system are as usual in Oligochaeta. The supra-oesophageal 

 ganglia lie in the third somite ; the first pair of the ventral 

 ganglionic chain, in the fourth. Several large nerves are given 

 off from the supra-oesophageal ganglia and the oesophageal 

 ring, and go to supply the buccal muscles as well as the pros- 

 tomium and the body walls of the first two somites. The first 

 pair of ventral ganglia send large nerves forward to the ventral 

 wall of the second and third somites. The ventral nerve cord 

 contains the usual tubular fibres, and is surrounded by a mus- 

 cular sheath containing a copious blood supply derived from 

 the sub-neural trunk. Nerves are given off laterally to the 

 body walls in each somite; they penetrate the longitudinal 



