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. Discophora. We now come to medusae 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- 

 sce, represented by the genera 

 jEgineta, 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, ClMrybdcea, the radiating 

 canals are branched, thus con- 

 necting this group with the true 

 covered-eyed Acalephs, such as Aurelia. 



0. and R. Hertwig have fully confirmed Haeckel'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 



Fig. 43.-J 

 , front view. 



