PHYLUM CCELENTERATA 143 



veloped mesoderm. Between these layers is a jelly-like non- 

 cellular substance, the mesoglea. The body-wall encloses a 

 single cavity, the ccelenteron or gastrovascular cavity, in which 

 both digestion and circulation take place. In some of the cce- 

 lenterates, like Hydra (Fig. 65), this cavity is simple, but in 

 others, like Aurelia (Fig. 79), it is modified so as to include 

 numerous pouches and branching canals. 



The two principal types of ccelenterates are the polyp or 

 li yd raid, and the jellyfish or medusa. These are fundamentally 

 similar in structure (Fig. 76), but are variously modified (Tables 

 V and VI). Both polyps and medusae are radially symmetrical. 



So far as is known, all ccelenterates possess stinging cells 

 called nematocysts ; these are organs of offense and defense. 

 Muscle fibrils are present in a more or less concentrated con- 

 dition. Nerve-fibers and sensory organs are characteristic 

 structures; they may be few in number and scattered as in 

 Hydra (p. 112), or numerous and concentrated as in Aurelia 

 (p. 131, Fig. 80). 



Physiology. - - The food of ccelenterates consists principally 

 of small, free-swimming animals, which are usually captured by 

 means of nematocysts and carried into the mouth by tentacles 

 and cilia. Digestion is mainly extracellular, enzymes being dis- 

 charged into the gastrovascular cavities for this purpose. 

 The digested food is transported to various parts of the body by 

 currents in the gastrovascular cavity, and is then taken up by 

 the entoderm cells and passed over to the ectoderm cells. Both 

 respiration and excretion are performed by the general surface 

 of the ectoderm and entoderm. Motion is made possible by 

 muscle fibrils, and many species have also the power of loco- 

 motion. There is no true skeleton, although the stony masses 

 built up by coral polyps support the soft tissues to a certain ex- 

 tent. The nervous tissue and sensory organs provide for the 

 perception of various kinds of stimuli and the conduction of im- 

 pulses from one part of the body to another. Ccelenterates 

 are generally sensitive to light intensities, to changes in the 



