20 JELLY-FISH, STAR-FISH, AND SEA-URCHINS. 
unite themselves with the elements of the nerve- 
rings. There are also described several peculiar 
tissue elements, such as, in the umbrella, nerve- 
fibres 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 agere- 
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 Medusze 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. 
