REPORT ON THE NEMERTEA. 73 



NERVOUS SYSTEM. 



With respect to the nervous system, I cam indebted to the Challenger collection for 

 very valuable additional data. In former publications ( IX, X ) I have dwelt at length 

 on the peculiar arrangement of nervous tissue in the Nemertea as I had found it to exist 

 in specimens that were obtained at Naples ; I am now enabled to give a more exhaustive 

 description of this important system, and will commence by a short account of certain 

 points in the latest investigations into the nervous apparatus of the lower forms of animal 

 life, in order the better to explain the bearing upon questions of general morphology 

 which the arrangements as we find them in the Nemertea may happen to have. 



The general and important conclusions arrived at by Kleinenberg in his classical 

 Memoir on Hydra, conclusions which have since found their way into handbooks and 

 textbooks as Kleinenberg's Theory of the neuro-muscular cells, have of late years been 

 emendated by 0. and E. Hertwig. These investigators have propounded a general hypo- 

 thesis on the phylogenetic development of the nervous system, which in their treatise 

 Das Nervensystem und die Sinnesorgane der Medusen (Leipzig, 1878), is formulated 

 (p. 170) as follows : — 



" We assume that in all Metazoa the ectoderm from which the (animal) nervous 

 system, with its motor and sensory terminal apparatus, has originated, was primitively 

 constituted of a simple layer of homogeneous cells in the same way as may be 

 noticed everywhere in the earliest ontogenetic stages. We further assume that these 

 cells, or at least part of them, have at an early period entered into mutual connection 

 by protoplasmic processes, and have thus formed a more closely connected cell-stratum. 

 According to our hypothesis, and on the principle of division of labour between the cells 

 thus connected, there has been gradually developed a primitive nerve system out of 

 this connected stratum. Whilst certain of these cells secreted contractile substance, 

 others were provided on their surface with tactile hairs, and a third set acquired very 

 numerous connections, the simple epithelium cells of the one-layered ectoderm thus 

 becoming gradually and more or less simultaneously differentiated into epithelial muscle- 

 cells, sense-cells, and ganglion-cells. Their protoplasmic connections, modified into 

 specific nerve substance, have pari passu become converted into a plexus of nerve fibrils. 

 When, later on, the ectoderm became constituted of more than one layer, the ganglion- 

 cells were the first (of all the three elements just mentioned) to separate from the 

 surface epithelium and to acquire a deeper situation." 



Balfour, in his Comparative Embryology (vol. ii. p. 333), accepts the leading features of 

 this important hypothesis, partly substituting it for the earlier suggestion of Kleinenberg. 



The latter, in his latest publication, 1 revindicates his original theory against the 



1 Zeitschr. f. wiss. Z00L, Bd. xliv. p. 204. 

 (ZOOL. CHALL. EXP. — PART LIV. — 1887.) IHlll 10 



