REPORT ON THE NEMERTEA. 79 



independent from those of the brain commissure that is seen to pass under it, and to 

 have a different texture and arrangement. 



Our observations on the nerve-plexus would not be complete if we did not allude to 

 the very elaborate branches that pass out from it into the superposed muscular layers 

 which they innervate. Some of them can even be traced as thick radial nerves piercing 

 these muscles, and spreading out into the integument (PL XIII. fig. 6, n). Similarly the 

 underlying muscular layers receive fine nerve-twigs out of the plexus, which are thus 

 directed inwards as well, and first penetrate into the circular layer /3. For this reason 

 they are best seen in longitudinal sections. The peripheral nerve system of the Schizo- 

 nemertea has thus — as was already fully indicated in a former publication (x) — a totally 

 different character from that of the Hoplonemertea. The profusion of radial nerve-stems 

 springing from the plexus, every transverse section showing a great number of them, may 

 convince us of the high degree of elaboration to which the nerve system of this group 

 attains, and of which the great sensitiveness and quickly reacting movements of the worms 

 themselves are the outwardly visible tokens. 



Nor may we omit to record the important fact, which was first observed in a Challenger 

 specimen of Cerebratulus corrugatus, that in the region of the long slit-like mouth 

 and oesophagus (behind the region where the very strong nerve, to which the name of 

 vagus-nerve has been given (V, IX), leaves the inferior brain-lobes on its way to inner- 

 vate the oesophagus) we can observe that from the plexus distinct nerves become detached, 

 pierce the circular and inner longitudinal muscle layers (/3 and a), cross the circum- 

 cesophageal blood-space and enter the tissue of the wall of the blood lacunae and of the 

 oesophagus to assist in innervating these important organs. The morphological significance 

 of this fact will be further insisted upon later on (cf. pp. 134, 142). The phenomenon 

 is figured on PL XIV. figs. 3, 4. 



We have now traced the facts concerning the plexus and the medullary nerve. In a 

 general way these descriptions may be said to be applicable to the plexus of Carinina, 

 which, however, as was already noticed, is a less favourable object for study. It would 

 seem as if in this species the nervous tissue, passing inwards amidst the muscles, again 

 spreads out into a second plexiform arrangement between the muscular layers a and 8. 

 This phenomenon, however, requires confirmation in more specimens than the two that 

 have been available for the present investigation. 



One point alone requires a few words of further elucidation before we can pass from 

 the nerve plexus to another paragraph, viz., the question as to whether the name of 

 proboscidian sheath-nerve, formerly given by me to what I now propose to call the 

 medullary nerve or the Nemertean medulla, must for the future be dropped altogether. 

 It certainly must, if we wish to retain it for the longitudinal nerve originally so called ; 

 but, curiously enough, I have now been able to make out the presence of another longi- 

 tudinal nerve to which the name may very properly apply. 



