62 INVERTEBRATE ANIMALS. 
CH ANP TES Ver 
DIVISIONS OF THE HYDROZOA. 
Tue Hydrozoa are divided into four great divisions, each of 
which requires some notice, as presenting points of special in- 
terest. These divisions or sub-classes are known by the names 
of Hydroida, Siphonophora, Discophora, and Lucernarida. 
SUB-CLASS HYDROIDA. 
This sub-class comprises all the sea-firs and their allies, com- 
monly known to naturalists as the “ Hydroid zoophytes,” from 
their resemblance to the fresh-water polype (ydra), which is 
also a member of this division. The Hydrodda are defined by 
the fact that they consist of an alimentary region or “ polypite,” 
which is furnished with a mouth and prehensile tentacles at tts 
distal end, and with an adherent disc at its proximal extremity. 
In some few cases the hydrosoma consists of but one such 
polypite (as in the Aydrida and some of the Corynida) ; but 
generally the hydrosoma is composed of a greater or less 
number of similar polypites all united by a coenosarc or com- 
mon trunk (as in the majority of the Coryzdda, and in the 
Sertularidaand Campanularida). Inthe great majority of cases, 
also, the hydrosoma is not unattached, but is fixed to some solid 
object by its proximal extremity or hydrorhiza. The Hydroid 
zoophytes exhibit three principal types of structure, which con- 
stitute so many orders. 
OrpER I. Hypripa.—In the first order we have only the 
well-known fresh-water polypes or Hydra, of which we may 
take the common green Hydra (H. viridis) as the type. This 
singular little creature may be found with a little trouble in 
most of our streams and pools, and it is quite visible to the 
