COMPARATIVE ZOOLOGY. 
246 
A few Sponges, as the Venus’s Flower-basket (. Euplec- 
tella), have siliceous and others have calcareous skeletons. 
Excepting a few small fresh-water species (as Spon- 
gilla ), Sponges are marine. In the former, the cellular 
part is greenish, containing chlorophyll; in the latter, it 
is brown, red, or purple. In preparing the Sponge of 
commerce, this is rotted by exposure, and washed out. 
The best fishing-grounds are the eastern end of the Medi¬ 
terranean and around the Bahama Islands. 
Subkingdom III. —Ccelenterata. 
These radiate animals are distinguished by having a bpdy 
cavity, whose walls have, at least, two layers of cellular 
tissue, an outer (ectoderm) and inner ( endoderm ), and usual¬ 
ly a middle layer (mesoderm), this cavity serving for both 
digestion and circulation. They have thread-cells, minute 
sacs containing a fluid, and connected with barbed fila¬ 
ments capable of being thrown out for stinging purposes. 
Most are provided with hollow tentacles around the mouth. 
All are aquatic, and nearly all are marine. There are three 
classes, represented by the Hydra, Sea-anemone, and Cte- 
nophores. All reproduce by eggs, and the first two also 
by budding. 
Class I.— Hydrozoa. 
These Coelenterates have no separate digestive sac, so 
that the body is a simple tube, or cavity, into which the 
mouth opens. The nervous system is slightly developed. 
Such are the fresh-water Hydra and the oceanic Jelly-fish 
(Acaleph or Medusa). 
The body of the Hydra is tubular, soft, and sensitive, 
of a greenish or brownish color, and seldom over half an 
inch long. It is found spontaneously attached by one 
end to submerged plants, while the free end contains the 
orifice, or month, crowned with tentacles, by which the 
creature feeds and creeps. The body-wall consists of two 
cellular layers—ectoderm and endoderm. These surround 
