218 THE ORGANIZATION OF 



halves (/r 3 ) of the ring; and the main recurrent vessel, (/*",) along 

 the lower middle line of the body, standing at the junction of 

 the vessels which come from the proboscis. 



The nervous system has the same apparently complicated dis- 

 tribution as the circulatory system, but it is even more simple 

 than the latter. The main portion of it is a thick string () 

 which rests on the lower middle line of the body, below all the 

 other organs, and extends, with scarcely a diminution in its 

 thickness, from the mouth to the posterior end of the intestine, 

 (from g to g 3 ). At numerous points along its whole length it 

 gives off at right angles, on each side, parallel twigs (g- 1 ) which 

 taper into slender threads and bury themselves in the thickness 

 of the highly muscular skin. According to Lacaze Duthiers, 

 from whose figures the diagrams are constructed, there are no 

 ganglionic swellings where these twigs are given off; but yet, 

 inasmuch as we find, in most of the Articulata, such swellings 

 where branches diverge, I think I shall not err if I assume that 

 we have what are essentially the same in Bonellia. Beside these 

 lateral twigs, there are others which branch over the various or- 

 gans. That part of the system which corresponds to the so-called 

 brain and nervous collar of the higher groups, is most singularly 

 disguised in this animal, and is used, it would seern, merely for the 

 purpose of touch. The part in question arises from the anterior 

 end (g-) of the main ventral cord, and passes, on each side of the 

 throat, into the proboscis. There the two branches (?,*) follow 

 its margin, just within, and below the level of, the line of vessels 

 (A 3 , A 4 , /i 5 ) which run there, and meet along the front of the trans- 

 verse projections. The latter are said to be extremely sensitive to 

 touch, and apparently in accordance with this, the nervous cord 

 sends off innumerable minute threads which penetrate the skin 

 in the same way that they do in the tactile organs of other ani- 

 mals. Throughout the whole length and breadth of the probos- 

 cis the nervous collar preserves a uniform thickness, which is 

 less than one half of that of the main ventral cord (g-) ; and there 

 is no part of it which might be called the brain, properly speak- 

 ing; unless we judge that to be it, from its position, which lies at 



