ZOOLOGY. 



B 



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 sei rations 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. Discophora. We now come to medusas which 



differ from the Hydromedusae 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 

 jEgimta, 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, C7i(tr//b(l(ea, the radiating 

 canals are branched, thus con- 

 necting this group with the true 

 covered-eyed Acalephs, such as Aurdia. 



0. and R. Hertwig have fully confirmed Ilaeckel's discov- 

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

 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 



C 



D 



Fig. 4%.31onoffrap>iix -prio'lon. 

 e, front view. After Nicholson. 



