THE NERVOUS SYSTEM. 63 



probable that they are the channels by which impulses affect- 

 ing any of the cells of the ectoderm are conveyed to other 

 cells and excite their contraction. 



The researches of Eimer 1 upon the nervous system of the 

 Ctenophora are in perfect accordance with this view. The 

 mesoderm is traversed in all directions by very fine fibrils, 

 varying in diameter from 3o ^ 00 to T2 | 00 of an inch. These 

 fibrils present numerous minute varicosities, and, at intervals, 

 larger swellings which contain nuclei, each with a large and 

 strongly refracting nucleolus. These fibrils take a straight 

 course, branch dichotomously, and end in still finer filaments, 

 which also divide, but become no smaller. They terminate 

 partly in ganglionic cells, partly in muscular fibres, partly in 

 the cells of the ectoderm and endoderm. Many of the nerve- 

 fibrils take a longitudinal course beneath the centre of each 

 series of paddles, and these are accompanied by ganglionic 

 cells, which become particularly abundant toward the aboral 

 end of each series. The eight bands meet in a central tract 

 at the aboral pole of the body; but Eimer doubts the nervous 

 nature of the cellular mass which lies beneath the lithocyst 

 and supports the eye-spots. 



The nervous system of the Ctenophoran is, therefore, just 

 such as would arise in Hydra, if the development of a thick 

 mesoderm gave rise to the separation and elongation of 

 Kleinenberg's fibres, and if special bands of such fibres, 

 developed in relation with the chief organs of locomotion, 

 united in a central tract directly connected with the higher 

 sensory organs. We have here, in short, virtual, though in- 

 completely differentiated, brain and nerves. 



All recent investigation tends more and more completely 

 to establish the following conclusions : firstly, that the central 

 ganglia of the nervous system in all animals are derived from 

 the ectoderm; secondly, that all the nerves of the sensory 

 organs terminate in cells of the ectoderm ; thirdly, that all 

 motor nerves end in the substance of the muscular fibres to 

 which they are distributed. So that, in the highest animals, 

 the nervous system is essentially similar to that of the lowest ; 

 the difference consisting, in part, in the proportional size of 

 the nerve-centres, and, in part, in the gathering together of 

 the internuncial filaments into bundles, having a definite 

 arrangement, which are the nerves, in the ordinary anatomical 

 sense of the term. 



1 " Zoologisclie Studien auf Capri." Leipsic, 1873. 



