SENSE-ORGANS IN MEDUSAE 
765 
of the marginal body in the sensory niche a pair of cone-shaped 
swollen thickenings of the ectoderm which enclose under the 
epithelium a thick layer of ganglion cells and nerve fibers." He 
says, moreover, that the nervous system in Chr3^saora has the 
same general structure as in Aurelia. These statements appar- 
ently refer to the structures that I have described, but fail to 
give a correct idea of them. As already stated, von Lendenfeld 
found in Crambessa two prominences on the sides of the niche 
which he homologizes with the cone-shaped thickenings of Claus. 
But as he distinctly says that they are not invaginations, they 
must be quite different from the organs to which I suppose Claus 
alludes. Nothing need be added concerning the histology of 
the endodermal parts of the rhopalium at this stage except that 
the cells have increased in depth. 
THE ADULT PELAGIA 
The brothers Hertwig have given in their work on the nervous 
system and sense-organs of the medusae ('78 a, p. 109) a very clear 
account of the position and structure of the rhopalia in Pelagia 
noctiluca (Per. Les.) and have described their development from 
the ephyra stage. 
This agrees in the main with what I have said of the rhopalium 
of Chrysaora except that the rhopalial canal in the adult does not 
penetrate the mass of concretions. The ephyra of Pelagia agrees 
in every essential particular with the same stage in Chrysaora. As 
in Chrysaora at this stage, the gastric diverticulum does not pene- 
trate the rhopalium but its interior is filled with" endoderm cells, 
and the rhopalium is not surrounded by a sensory niche. The 
most important events which take place in the development from 
this to the adult stage are, according to the Hertwigs, the hol- 
lowing out of the rhopalium, an increase in the number of con- 
cretions, and the formation of the sensory niche by the outgrowth 
of the hood and free edges of the marginal lobes. 
I have studied only the adult sense-organ in Pelagia cyanella 
(Per. Les.) and can confirm the description of the adult given by 
