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 or bit of wood or aquatic plant, is a simple cylinder 

 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 



