PHYLUM COELENTERATA 153 



Many of the animals used as food have hard skeletal parts that 

 will not digest. These indigestible portions are ejected from the 

 cavity through the mouth by reverse peristalsis, and the process is 

 known as egestion. Eespiration furnishes the necessary exchange of 

 oxygen and carbon dioxide by diffusion through the plasma mem- 

 branes. The dissolved oxygen in the water in which the animal lives 

 is the source of this element. 



Catabolism or dissimilation takes place in the protoplasm and 

 involves the union of oxygen with the substance of the protoplasm 

 to transform potential energy there to kinetic energy and heat. 

 Accompanying this oxidation there are produced some waste by- 

 products in solution including urea, uric acid, and water which 

 must be expelled from the body. In hydra this excretion is accom- 

 plished by diffusion through the general surface of the body. There 

 is some indication that there may be accumulation of waste prod- 

 ucts in endoderm cells as cytoplasmic granules, which finally escape 

 through the gastrovascular cavity and mouth. It will be noticed 

 that these phases of metabolism are, in general, very similar to the 

 comparable processes in Protozoa and the same similarity will be 

 noticed when they are compared later with the higher forms of 

 animals, because the protoplasmic requirements are the same in all 

 animals. 



The Nervous System and Nervous Conduction 



The neuroepithelial cells are distributed among the other cells of 

 the germ layers. There is a greater abundance of them on the hypo- 

 stome, basal disc, and tentacles than along the length of the column. 

 The greatest concentration of these cells is in the hypostome around 

 the mouth, which makes this region in a sense comparable to a primi- 

 tive brain. These cells all over the body are in contact with each 

 other by means of their processes forming what is called a nerve net. 

 When one sensory cell is stimulated, all of the sensory cells seem to 

 be stimulated in some degree. A sufficiently strong stimulus affecting 

 any sensitive point will stimulate the entire body. This is a definite 

 organized type of nervous system but not a very efficient one. 



Reproduction and Life Cycle 



Reproduction is both asexual and sexual. Asexual reproduction 

 is accomplished very efiiciently and quite rapidly. This process is 

 essentially reproduction by somatic cell division. Nutrient mate- 



