AN OLIGOCHiETE WORM. 



307 



JVervous System. — The brain is dorsal in segment 1. Anteriorly 

 it gives off two connectives which unite ventrally to form the 

 ventral nerve-cord. The connectives can be traced in sections ; 

 they are in close approximation with the body-wall of the 1st 

 segment and seem to be connected at this point with a ring 

 of tissue which appears to be nervous. It may be this nervous 

 ring of tissue which makes the 1st segment, including the 

 prostomium, so extremely sensitive. 



The brain is twice as long as broad when stretched to its 

 fullest extent, and oval in shape. Posteriorly it is rounded, but 

 occasionally shows signs of a concavity. The brain itself seems 

 to be composed of two parts, a central core and an outer portion 

 (PI. XLYI. figs. 1 & 2). The outer portion is cellular in structure ; 



Text-fig. 62. 



Anterior blood-sjstem of TSnchytrceus pellucidiis. 

 d.v., dorsal vessel ; v.v., ventral vessel. 



the cells are small and numerous, and each one has a prominent 

 nucleus. In preparations, the outer portion stains very deeply 

 owing to the large number of nuclei. The central core is 

 fibrous in structure with no nuclei showing. 



Pigment-spots in the Brain. — At the posterior end of the brain 

 there are numerous black spots in the outer cellular portion 

 (fig. 2). These vary in number, usually 4 to 8, and are not 

 regularly or symmetrically placed. Although these bodies have 

 been mentioned by some authors, I have never seen them fully 

 described, nor have I seen any suggestions as to their possible 

 function. If the brain be carefully dissected out and examined 

 under the high power of a microscope, the pigment-spots seem to 

 be dark, granular or oily, with a clear spot in the centre. They 

 are always present in this worm. Each black spot is composed 



