EEPOET ON THE NEMERTEA. 87 



conspicuous than in Carinina, where we have already pointed out the apj^arent absence 

 of any sharp or distinct boundary Une between the cellular brain- tissue and the sur- 

 rounding cellular tissue of the integument. 



We cannot pass on to the description of the perijiheral nerves without first referring 

 to the terminal portion of the lateral nerve-stems, known to terminate at the posterior 

 end of the body, right and left of the anus in the Schizonemertea, but also known to 

 meet in a connecting commissure above the anus in several Hoplonemertea (IX). This 

 commissure was found by me in several Challenger species, but at the same time I was 

 able to verify the unexpected fact that in Eupolia the fibrous cores of the longitudinal 

 nerve-stems are also posteriorly united by a commissure. What most especially 

 deserves attention in this posterior commissure of Eupolia is, that it is found helow the 

 anus, the longitudinal stems and the commissure, together with the brain, thus forming 

 an immensely elongated ring round the intestine, whereas in most of the Hoplonemertea 

 alluded to, all the portions of the nerve-system may be said to remain above the intes- 

 tine. This is, indeed, very emphatically the case in Amphiporus moseleyi, where we find 

 (PI. IX. fig. 4) not only the brain and the anal commissure above the intestine, but also 

 the longitudinal stems, that take their course above the intestinal caeca. Nevertheless, 

 in Drepanophorus the anal commissure is above the intestine, although here the 

 longitudinal stems are diametrically opposite in position, i.e., below the intestinal caeca. 

 They were for this reason considered (IX) to furnish a transition stage to the ventral cord 

 and circumoesophageal ring of Annelids and Arthropods, a consideration which derives 

 very strong support from the existence of transverse commissures that will hereafter be 

 described. At all events, these very curious difi'erences — the anal commissure of Eupolia 

 is figured on PI. VII. fig. 8 — furnish another proof of the extraordinary plasticity which 

 we meet with in the group of the Nemertea, with respect to the morphology of the most 

 important com^^onents of the system ; a plasticity and diversity which are at the same 

 time indicative of the primitive and low scale on which the Nemertea may be said to 

 find themselves. 



Coming now to the, peripheral nerve-system, I may note that I have already, some 

 years ago (X), stated that it is difiicult to apply this name in its generally accepted 

 significance to the arrangement which we find in Schizonemertea and in Palseonemertea. 

 It is, however, applicable to that of the Hoplonemertea. Here only we find distinct 

 metamerical peripheral nerves leaving the longitudinal nerve-stems at regular intervals, 

 and innervating the body musculature, the integument, the internal organs, &c. 



In the Schizonemertea and Pala^onemertea the cephalic nerves, starting from the brain, 

 are directly comparable with those of the Hoplonemertea, but the rest of the peripheral 

 system is here represented by the plexus and its innumerable branches and twigs, which 

 are directed upwards and downwards, serve for the same purpose, and render the peripheral 

 arrangement in this group so primitive and so important. Still, in very large specimens 



