CCRLENTERATA. 237 
Venus flower-basket (Z’uplectella); but the majority have 
a fibrous, horny skeleton, which in some forms is strength- 
ened by siliceous needles (spzcula). Excepting a few small 
fresh-water species (as Spongilla), Sponges are marine. In 
the former, the gelatinous part is greenish; in the latter, it 
is brown, red, or purple. In preparing the sponge of com- 
merce, this is rotted by exposure, and washed out. The 
best fishing- grounds are the eastern end of the Mediter- 
ranean and around the Bahama Islands. 
Subkingdom.—Cc.LenTeRATA. 
These radiate animals are distinguished by having a 
distinct body-cavity, the walls of it consisting of two lay- 
ers of cellular tissue, an outer (ectoderm) and inner (endo- 
derm); and thread cells, which are minute sacs contain- 
ing a fluid, and connected with barbed filaments capable 
of being thrown out for stinging purposes. Most are pro- 
vided with hollow tentacles around the mouth. All are 
aquatic, and nearly all are marine. There are two classes, 
represented by the Hydra and Sea-anemone. Both repro- 
duce by budding and by eggs; but in the former the eggs 
are developed from the exterior of the body, while in the 
latter they are internal. 
Cuiass I.—Hydrozoa. 
These Coelenterates have no separate digestive sac, so 
that the body is a simple tube, or cavity, into which the 
mouth opens. A nervous system is not apparent. Such 
are the fresh-water //ydra aud the oceanic Jelly - fish 
(Acaleph or Medusa). 
The body of the Hydra is tubular, soft, and sensitive, 
of a greenish or reddish 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 mouth, crowned with tentacles, by which 
