20 JELLY-FISH, STAR-FISH, AND SEA-URCHINS. 



unite themselves with the elements of the nerve- 

 lings. There are also described several peculiar 

 tissue elements, such as, in the umbrella, nerve- 

 fibiX3S which probably stand in connection with 

 epithelium-cells ; nerve-cells which pass into muscle- 

 fibres, similar to those which Kleinenberg has 

 called neuro-muscular cells ; and, in the tentacles, 

 neuro-muscular cells joined with cells of special 

 sensation (Sinneszellen). 



No nervous elements could be detected in the 

 convex surface of the umbrella, and it is doubtful 

 whether they occur in the veil. 



In some species the nerve-fibres become aggre- 

 gated in the region of the generative organs, and 

 in that of the radial canals, thus giving rise in these 

 localities to what may be called nerve-trunks. But 

 in other species no such aggregations are apparent, 

 the nervous plexus spreading out in the form of an 

 even trellis-work. 



In the covered- eyed MedusiB the central nervous 

 system consists of a series of separate centres which 

 are not connected by any commissures. These 

 nerve-centres are situated in the margin of the 

 umbrella, and are generally eight in number, more 

 rarely twelve, and in some species sixteen. They 

 are thickenings of the ectoderm, which either 

 enclose the bases of the sense-organs, or only cover 

 the ventral side of the same. Histologically they 

 consist of cells of special sensation, together with 

 a thick layer of slender nerve-fibres. Ganglion- 

 cells, however, are absent, so that the nerve-fibres 

 are merely processes of epithelium- cells. 



