ME. G. J. ROMANES ON THE LOCOMOTOR SYSTEM OF MEDUSAS. 201 
siderable thickness which contrast in a striking manner with the almost immeasurably 
slender fibres of the upper nerve-ring. A second point of diiference consists in the 
surprising wealth of ganglion cells in the one ring as compared with the other. 
Thus, on the whole, there is no doubt that the lower nerve-ring presents a higher 
grade of structure than does the upper, as shown not only by the greater multiplicity 
of nerve-cells and fibres, but also by the relation in which these elements stand to the 
epithelium. For in the case of the lower nerve-ring the presumably primitive con¬ 
nexions of the nervous elements with the epithelium is well-nigh dissolved—this 
nerve-ring having thus separated itself from its parent structure, and formed for itself 
an independent layer beneath the epithelium. The two nerve-rings are separated 
from one another by a very thin membrane, which, in some species at all events, is 
bored through by strands of nerve-fibres which serve to connect the two nerve-rings 
with one another. 
The peripheral nervous system is also situated in the ectoderm, and springs from 
the central nervous system, not by any observable nerve-trunks, but directly as a 
nervous plexus composed both of cells and fibres. Such a nervous plexus admits of 
being detected in the subumbrella of all Medusae, and in some species may be traced 
also into the tentacles. It invariably lies between the layer of muscle fibre and that 
of the epithelium. The processes of neighbouring ganglion-cells in the plexus either 
coalesce or dwindle in their course to small fibres : at the margin of the umbrella 
these 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 hi connexion 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 aggregated 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 Medusse 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. 
Drs. Hertwig made no observations on the peripheral nervous system of the 
covered-eyed Medusse ; but they do not doubt that such a system would admit of 
MDCCCLXXX. 2 D 
