62 ZOOLOGY, 
side; in G. pristis the hydrosome is brcader, more lanceo- 
late, and the sharp, tooth-like cells are arranged on both 
sides of a median stem. In Phyllograptus typus the hy- 
drosome is broad and oval, leaf-like, the serrations of the 
leaf marking off the cells, which are apparently supported 
on acentral axis. The group also has some affinities to the 
Poiyzoa, and is probably a generalized or synthetic type of 
animals, 
Order 2. Discophora.—We now come to meduse which 
differ from the Hydromeduse in 
developing directly from eggs; 
in having usually no velum ; with 
branching gastro-vascular canals, 
and covered sense-organs. They 
intergrade, however, with the 
Hydrovidea by the members of the 
group or sub-order Trachymedu- 
s@, represented by the genera 
Aigineta, Geryonia, etc. These 
are small jelly-fishes, with often 
a remarkably long proboscis 
(manubrium), as in Geryonia, 
and with either four single radi- 
ating canals, or, in addition, as 
in Geryonia, a number of smaller 
canals on the edge of the disk ; 
or, as in a still more complicated 
form, Charybdwa, the radiating 
0k eee eeNhdeon” canals are branched, thus con- 
" uneatine this group with the true 
covered-eyed Acalepks, such as Aurelia. 
O. and R. Hertwig have fully confirmed Haeckel’s discov- 
ery of the nature of the nervous system in the Geryonide. 
They find that the nervous system is developed in the ecto- 
derm and consists of two ‘‘ ring-nerves” around the edge 
of the disk, formed of two filaments, one lying on the upper, 
the other on the under side of the velum, immediately at its 
insertion. From this double nervous ring filaments are sent 
off to the ganglia near the sense-organs. This sort of a 
