16 JELLY-FISH, STAR-FISH, AND SEA-URCHINS. 
Medusze, we have a most important step in the 
histology of these animals. Haeckel’s results in 
these respects have since been confirmed by Claus, 
“Grundziige der Zoologie,” 1872; Allman, “A 
Monograph of the Gymnoblastic or Tubularian 
Hydroids,” 1871; Harting, “ Notices Zoologiques,” 
Niedlandisches “Archiv. f. Zool.,” bd. i, Heft 3, 
1873; F. E. Schulze, “ Ueber den Bau von Syncorzne 
Sarsii”; O. and R. Hertwig, “Das Nervensystem 
und die Sinnesorgane der Medusen.” 
The last-named monograph is much the most 
important that has appeared upon the histology of 
the Meduse. I shall, therefore, give a condensed 
epitome of the leading results which it has estab- 
lished. 
There is so great a difference between the nervous 
system of the naked and of the covered eyed 
Medusez, that a simultaneous description of the 
nervous system in both groups is not by these 
authors considered practicable. Beginning, there- 
fore, with the naked-eyed division, they describe 
the nervous system as here consisting of two parts, 
a central and a peripheral. The central part is 
localized 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 many species the upper nerve-ring 
is spread out in the form of a flattish layer, which 
* This is the name given to a small annular sheet of tissue 
which forms a kind of floor to the orifice of the swimming-bell, 
through the central opening of which floor the manubrium passes. 
The structure is shown in Fig. 1. 
