52 ZOOLOGY. 
part formed by the digestive tract. The Celenterata pre- 
sent striking examples of self-division, gemmation, and 
alternate generations, and very great extremes in degree of 
complexity of structure. 
The different groups have a high geological antiquity; 
the species of Hydroid and coral-polyps serving as time- 
marks to measure off geological periods. 
Crass I.—Hyprozoa (Hydroids and Acalephs.) 
General Characters of Hydrozoa.—An excellent idea of 
the general structure of the Hydrozoa may be obtained from 
a study of Hydra, the type or example of the whole class, all 
the other forms being but a modification and elaboration of 
this simple type. The characters of the class as a whole are 
based on what is found to constitute the structure of 
Hydra. 
Order 1. Hydroidea.—The animal next higher in struc- 
ture than the sponge is the curious Profohydra discovered 
by Greef among diatoms and sea-weeds at Ostend. It is re- 
garded by Greef as the marine ancestral form of the Celen- 
terates. It is the simplest Coelenterate yet discovered. As 
the form of the fresh-water Hydra is familiar, Protohydra 
may be best described us being similar to that, except that 
it is entirely wanting in tentacles. It is made up of two 
layers (an ectoderm and endoderm, no mesoderm having yet 
been discovered), with a mouth and stomach (gastro-vascular 
cavity). 
A more complicated form is the fresh-water Hydra, which 
is commonly found on the under side of the leaves of aquatic 
plants. ‘There are two varieties of Hydra vulgaris appar- 
ently common to the fresh waters of the old and new world ; 
they are Hydra viridis and fusca. The somewhat club- 
shaped body consists of two layers, the inner (endoderm) 
lining the general cavity of the body, which serves both aa 
mouth and stomach, as well as for the circulation of the 
nutritive fluid, and is called the gastro-vascular cavity. 
The mouth is surrounded with-from five to eight tentacles, 
