262 PRINCIPLES OF ANIMAL BIOLOGY 



A curious feature of the development of sponges is their "inside-out" 

 gastrulation. It is the cihated cells of the blastula that are invaginated 

 and form the endoderm, whereas other gastrulas, if ciliated at all, regu- 

 larly bear the cilia on the outside. Sponges also have remarkable powers 

 of regeneration. Their bodies may be crushed, the separated cells sifted 

 through a bolting-cloth net upon a surface under water, and there the 

 cells gradually collect into lumps from which new sponges grow. 



In the irregular, spreading, fresh-water and bath sponges, there has 

 been some debate as to what constitutes the individual sponge. One 

 concept is that each osculum is the center of an individual, and that 

 the mass called a sponge is a colony. The boundaries of the individuals 

 would then necessarily be indefinite, since all the oscula are parts of one 

 system of canals. 



There are three classes of sponges: 



Class I. Calcarea. Sponges with spicules composed of calcium carbonate, mon- 

 axon or tetraxon in form. (Figs. 74, 139, 217.) / 



Order 1. Homocoela Order 2. Heterocoela 



Class II. Hexactinellida. Sponges with spicules composed of silicon, triaxon in 

 form. 



Class III. Demospongiae. Sponges with spicules composed of silicon, not triaxon 

 in form, or skeleton composed of spongin, or with skeleton of both spicules and 

 spongin. 



Order 1. Tetraxonida Order 3. Keratosa 



Order 2. Monaxonida 



Phylum 3. Coelenterata. — This phylum includes Hydra, the 

 hydroids, jellyfish, sea anemones, and corals (Figs. 218, 219). Its 

 members are radial in form and are all diploblastic. 

 They possess a coelenteron (page 101), a cavity with only 

 one opening, the mouth. There is no other body cavity. 

 They have tentacles, and in the ectoderm are stinging 

 cells used for offense and defense. Their nervous system 

 is very diffuse, consisting of a network of scattered cells. 

 While such a system provides for related actions through- 

 out the body, the coordination is often imperfect and 



Fig. 218. ,, , 



-Hydra, rather slow. 



with buds. There are in general two forms of body: (1) the polyp, 



Carolina Bio- which is typically tubular and elongated with tentacles 

 Logical Supply arouud One end, and (2) the medusa or ic^llvlish, which 

 is ordinarily compressed into a hemisphere or flat 

 disk with tentacles around the edge. Polyp and medusa are really 

 built on the same fundamental plan, as is readily understood if the 

 mouth and the center of the convex surface of the medusa be imagined 



