298 PHYSIOLOGY OF THE INVERTEBRATA. 



Prof. E. Haeckel* has described the nervous system of the 

 Geryonidm. ,It forms a circle all round the margin of the 

 nectocalyx (umbrella), "following the course of the radial 

 (nutrient) tubes throughout their entire length, and pro- 

 ceeding also to the tentacles and . marginal bodies." There 

 is a ganglion at the base of each tentacle from which the 

 above-mentioned nerves take their origin. These ganglia 

 contain fusiform and nucleated cells of high refractive 

 power. , " The nerves that emanate from the ganglia are 

 composed of a delicate and transparent tissue, in which no 

 cellular elements can be distinguished, but which is longitu- 

 dinally striated in a manner very suggestive of fibrillation. 

 Treatment with acetic acid, however, brings out distinct 

 nuclei in the case of the nerves that are situated in the 

 marginal vesicles, while in those that accompany the radial 

 canals, ganglion-cells are sometimes met with." Haeckel's 

 researches have been confirmed by Allman, Glaus, Harting, 

 Romanes, and others. 



According to Drs. 0. and R. Hertwig,t the nervous system 

 of the naked-eyed Medusm consists of two parts, a central 

 and a peripheral. " The central part is localised in the 

 margin of the swimming-bell, and there forms a nerve-ring, 

 which is divided by the insertion of the veil into an upper 

 and a lower nerve-ring. .... In all species the upper 

 nerve-ring lies entirely in the ectoderm. Its principal mass 

 is composed of nerve-fibres of wonderful tenuity, among 

 which are to be found sparsely scattered ganglion-cells. 

 .... The fibres which emanate from them are very deli- 

 cate, and, becoming mixed with others, do not admit of being 

 further traced." 



" Beneath the upper nerve-ring lies the lower nerve-ring. 

 It is inserted between the muscle-tissue - of the veil and 

 umbrella, in the midst of a broad strand wherein musole- 

 fibres are entirely absent." The lower nerve-ring belongs 



* Beitidge zur Naturgescliichte der Hydromedusen, 1865. 

 1' Das Nervensyatem und die iinnesorgane der Medusen. 



