63 



ZOOLOGY. 



B 



side ; in G. pristis the hydrosome is broader, more laflceo- 

 iate, 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 sxipported 

 on a central axis. The gronp also has some aflBnities to the 

 Polyzoa, and is probably a generalized or synthetic type of 

 animals. 



Order 2, DiscopJiora. — We now come to mednsae which 

 differ from the Hydromednsae in 

 developing directly from eggs : 

 in having nsaally no velum ; with 

 branching o-astro-Tascular canals, 

 and covered sense-organs. They 

 intergrade, however, with the 

 Hydroidea by the members of the 

 group or sub-order Trachymed'n- 

 scB, represented by the genera 

 ^gineta. 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, Cliaryljilirn, the radiating 

 canals are branched, thus con- 

 necting tills group with the true 

 covered-eyed Acalephs, such as Aureiia. 



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

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

 They find that the nervous system is developed in the ecto- 

 derm and consists of two •"ring-nerves" around the eda-e 

 of the disk, formed of two filaments, one lying on the upper, 

 the other on the under side of the velum, immediatclyat its 

 insertion. From this double nervous ring filaments are sent 

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



*, front view. — After XiclioLgon, 



