63 



ZOOLOGY. 



side ; in G. pristis the hydrosome is broader, 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 a central axis. The group also has some affinities to the 

 Polyzoa, and is probably a generalized or synthetic type of 

 animals. 



Order 2. Discopliora. — We now come to medusae which 

 differ from the Hydromedusse in 

 developing directly from eggs ; 

 in having usually no velum ; with 

 branching gastro-vascular canals, 

 and covered sense-organs. They 

 intergrade, however, with the 

 Hydroidea by the members of the 

 group or sub-order Tracliymedu- 

 scB, represented by the genera 

 ySgitieta, 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, Charybdma, the radiating 

 canals are branched, thus con- 

 necting this group with the true 

 covered-eyed Acalephs, such as Aurelia. 



0. and E. Hertwig have fully confirmed Haeckel's discov- 

 ery of the nature of the nervous system in the Geryonidm. 

 They find that the nervous system is develoi^ed 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. Prom this double nervous ring filaments are sent 

 off to the ganglia near the sense-organs. This sort of a 



C 



D 



Fig. iZ.—Monoqrapl'ns priodon. 

 c. front view.— After Nicholson. 



