REPORT ON THE NEMERTEA. 75 



For Vertebrates a plexiform arrangement is known to exist in the embryonic stages 

 of Amphibia, since the researches of Eemak and Strieker, and has lately been fully com- 

 mented upon by Gcette and Baldwin Spencer. 



The last writes •} — " There may be said to exist in the Amphibian embryo a complete 

 superficially-placed nervous sheath, out of which not only the central nervous system but 

 all the sense organs of both head and trunk are formed, and which gradually disappears 

 as these reach their full development." And further on : — " Aloug certain lines the cells 

 of the nervous layer proliferate, and it is by this proliferation that the rudiments of the 

 cranial nerves are laid down" (cf. p. 133). 



The significance of this plexiform arrangement of the embryonic Vertebrate nervous 

 system will be discussed in the chapter devoted to General Considerations, and also the peri- 

 pheral plexus of the adult Amphioxus, which lately has been more fully described by Eohon. 2 



Hence, since my former publications above cited, the necessity has grown more and 

 more obvious of not looking upon the brain-lobes and the lateral nerve-stems of the 

 Nemertea as the nervous system, but, though recognising their significance as more highly 

 developed centres, to admit the presence of a most complicated and intricate network of 

 nerve-tissue, originally — and in the more primitive species still — belonging to the 

 integument. This network is most fully developed in the Schizonemertea. In Carinina 

 its situation in the integument makes it more difficult to observe ; still I succeeded in 

 demonstrating it both here (cf. p. 54) and in the other Carinellidae. In the Hoplonemertea 

 the plexus has been replaced for the greater part by distinct nerves, of which the majority 

 show a metameric arrangement. 



We will now pass to a more detailed description of this network, thereby purposely 

 inverting the natural order by reserving the centres for the last. This apparent dis- 

 crepancy disappears, however, when we look upon the network as the most ancient 

 nervous arrangement, in which the centres have only gradually come forward. 



When once the eye has been trained by repeated observation to notice this particular 

 nervous tissue of the Nemertea, it is comparatively easy to distinguish it from the 

 surrounding tissue.- The peculiar punctate striation, the yellowish tint of the fibrous 

 elements, the very pale carmine hue of the nuclear ones, immediately reveal the presence 

 of nerve-tissue in sections, longitudinal or transverse, that have been made through 

 specimens stained with picrocarmine. 



And when we take for our starting point, and as a basis for further description, one 

 of the Schizonemertea of the Challenger, e.g., Cerehratulus corrugatus (PL XIV. figs. 3, 4 ; 

 PI. XIII. fig. 6, pi), we observe in all transverse sections that the two lateral nerve-stems 

 are in continuous connection with each other by nervous tissue that spreads out all round 

 the circular muscular layer /3, both dorsally and ventrally. Immediately outside of 



1 Some Notes on the Early Development of Eana temporaria, Quart. Jour Micr. Sci, Suppl., 1885. 



2 Denkschr. d. k. Akad. d. Wiss. Wien (math.-nat. CI.), vol. xlv. 



