THE LIFE OF THE SLIGHTLY COMPLEX ANIMALS 37 
living matter soon dies and decays, leaving the horny 
skeleton, which when cleaned and trimmed is ready for 
use. 
The most beautiful sponges are those with siliceous 
skeletons. The fine needles or threads of glass, arranged 
often in delicate and intricate pattern, make these sponges 
objects of real beauty. : 
21. Polyps, corals, and jelly-fishes——The general or typ- 
ical plan of body structure of those animals which come 
next in degree of complexity to the sponges can be best 
understood by imagining the typical cylindrical body of a 
sponge modified in the following way: The middle one 
of the three layers of the body wall not to be composed 
of cells in a gelatinous mass, but to be simply a thin non- 
cellular membrane; the body wall to be pierced by no 
fine openings or pores, so that the interior cavity of the 
body is connected with the outside only by the single 
large opening at the free end, and this opening to be sur- 
rounded by a circlet of arm-like processes or tentacles, 
continuations of the body wall and similarly composed. 
Such a body structure is the general or fundamental one 
for all polyps, corals, sea-anemones, and jelly-fishes. The 
variety in shape and the superficial modifications of this 
type-plan are many and striking; but, after all, the type- 
plan is recognizable throughout the whole of this great 
group of animals. Perhaps the simplest representative of 
the group is a tiny polyp which grows abundantly in the 
fresh-water streams and pools, and can be readily obtained 
for observation. It is called Hydra. 
22. Hydra.—The body of Hydra (Fig. 19), which is 
very small and appears to the unaided eye as a tiny white 
or greenish gelatinous particle attached to some submerged 
stone, bit of wood, or aquatic plant, is a simple cyiinder 
attached by one end to the stone or weed. The other free 
end is contracted so as to be conical, and it is narrowly 
open. Around the opening are six or eight small waving 
4 
