NERVOUS SYSTEM. 83 



The ganglia, according to M. de Quatrefages, are surrounded by a sheath forming a sort of 

 dura mater, at least in a large Borlasia {anglicel), for he could see none in the smaller species. 

 In the Enopla the muscular and other structures of the head form a somewhat condensed capsule 

 round the ganglia, independently of the delicate sheath proper of the nervous matter. The lon- 

 gitudinal fibres of the former, indeed, constitute powerful bands between the ganglia and the 

 inner muscular layer of the body- wall. This author also mentions the occurrence of ventricles in 

 the interior of the ganglia, and figures them in Folia berea ; such have not appeared in any 

 British form, though under pressure collections of fatty matter closely resemble his drawing. I 

 have never been able to see so many branches proceeding from the anterior borders of the 

 ganglia (as he shows) to the eyes, cephalic fossae, " mouth," and other tissues, in addition to the 

 great trunks and other branches posteriorly. The arrangement in the British Enopla (Plate XV, 

 fig. 4) is as follows : — Three very distinct branches occur on each side of the superior lobe 

 anteriorly (two about equal in size) ; a third, much smaller, to the outer side ; and, lastly, 

 traces of a fourth branch. The outline of the ganglion, throughout the rest of its extent, is quite 

 smooth. Various branches from these trunks proceed in the direction of the eyes; but the 

 nature of the cephalic tissues renders it very difficult to trace such an object as a pale nerve- 

 twig with certainty. Dr. Max Schultze gives a tolerably correct view of the ganglia and 

 nerves in Tetrastemma obscurum ; no branches, however, occur on the trunks in his figure. 

 This author, in a subsequent publication, founded one of the chief distinctions of his Enopla and 

 Anopla on the structure of the ganglia. E. Graeffe, again, in his remarks on a Tetrastemma 

 from Nice, states that he found a small cluster of otolite-capsules between the eyes, each capsule 

 containing a crowd of minute otolites. Professor Keferstein figures only two branches proceeding 

 from the anterior part of each superior lobe to the eyes in his Borlasia splendida {A. spectabilis, 

 De Quatrefages) ; but he represents a kind of meshwork, 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 to the 

 proboscis. I have not made out the latter in the British specimens. He also describes the 

 occurrence of an otolite or two in the middle of the ganglion of a young (Erstedia pallida. The 

 latter condition has been delineated by M. Claparede, the otolite-capsule in each case being situated 

 in the centre of the organ, and consisting of a cell-wall containing three spherical granules. The 

 same author figures the proboscis passing beneath the great or inferior nervous commissure, 

 with the central blood-vessel above both. 



b. Great Lateral Nerve-Trunks. 



These (n) spring from the inferior lobes of the cephalic ganglia, pass backwards 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 fibrous stroma of 

 the parts, and are often tinted of a reddish hue at their commencement. The branches given off 

 by these trunks are generally pale and indistinct, but by the use of dilute acetic acid in A. 

 lactifloreus, and in others without such aid, they can be satisfactorily observed. They are easily 

 seen, for instance, in A. pulcher, the reddish colour 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 this species (Plate XVI, fig. 3). In the same form, and in 

 N. JYeesii, there remains, even after continued pressure, a peculiar narrowing of the great trunks 



