BRITISH NEMERTEANS, AND SOME NEW BRITISH ANNELIDS. 359 



one of the chief distinctions of his Enopla and Anopla (Tremacephalidse and 

 Rhochmocephalidse) on the structure of the ganglia. Prof. Keferstein figures 

 only two branches, proceeding from the anterior part of each superior lobe to 

 the eyes in his Borlasia splendida, but he represents a kind of mesh-work, 

 formed by three or four trunks between the side of the lobe and the cephalic 

 sac, and a pair of nerves from the inferior commissure. No equivalent arrange- 

 ment to the two latter series has been seen in our species. M. Claparede* 

 figures the proboscis as passing beneath the great or inferior nervous commis- 

 sure in Prosorhochmiis Claparedii, and the central blood-vessel as placed above 

 both. 



The great nerve-trunks (w, in the various transverse sections), springing from 

 the inferior lobes of the cephalic ganglia, pass backwards in this group within 

 the inner (longitudinal) muscular layer of the body- wall to the posterior end of 

 the worm, where they terminate near the tip. They are surrounded by a coat 

 of the usual delicate fibroid stroma of the parts. The branches given off by 

 these trunks are generally pale and indistinct, but by the use of dilute acetic 

 acid in 0. alba, and in others without such aid, they can be satisfactorily observed. 

 They are easily seen, for instance, in 0. pulchra, the reddish hue which tinges 

 them at their commencement shining through the translucent integuments. An 

 elaborate plexus of branches from the lateral trunks has also been noticed in the 

 same species. In this form also there remains, even after continued pressure, a 

 peculiar narrowing of the great trunks immediately behind the ganglia, which, 

 if not an original condition, may be due either to comparative immunity from 

 pressure, or a tougher investment. The same constriction is seen in 0. purpurea. 

 In transverse section the nerves present a delicately granular appearance from 

 the ends of the cut fibres. No one who has seized on such specimens as 0. gra- 

 cilis in semi-contraction (though unwrinkled), and drawn them out to treble the 

 length and upwards, can doubt the peculiar elasticity that must pertain to the 

 lateral nerves in these animals, f 



The nerve-trunks were said by M. de Quatrefages to lie " between the external 

 longitudinal and internal transverse muscular fibres" of the body- wall ; a descrip- 

 tion which may in some respects apply to the Borlasians, but is inapplicable to 

 the present group. Frey and Leuckart J mention that the lateral trunks lie 

 to the inside of the muscular coats ; but while indicating the different arrange- 



* Beobachtungen, &c. pi. v. figs. 10 and 12. 



f The arrangement of the nervous system in the curious foreign Turbellarian, described under 

 the names of Bipalium, Stimpson and Grube, Spliyrocephalus, Schmarda, and Dunhpea, Perceval 

 Wright, presents a considerable variation from the foregoing, just as the external form of the head 

 and the digestive system do. Schmarda represents the cephalic ganglia as quite separated from 

 each other, except by connecting cords, and the great nerve-trunks placed close together in the 

 median line, with an intervening ganglion at regular distances. 



X Beitrage zur Kenntniss Wirb. Thiere, p. 72. 



