CHAPTER XVI 



HYDRA, OF PHYLUM COELENTERATA 



The phylum name, coelenterata (sel en ter ata), means ''hollow 

 intestine," and all of the representatives bear this out by possess- 

 ing a single large cavity in the body. There is a single opening to 

 this cavity, and it functions as both mouth and anus. There are 

 two general types of coelenterates ; the polyp form and the jellyfish 

 form. They are all modified gastrulas, have radial symmetry, and 

 possess tentacles with ''sting bodies" or nematocysts. Most of the 

 species are marine, but there are a few fresh-water forms. The 

 body wall is composed of two layers of cells, and for that reason 

 they are said to be diploUastic. These two layers are the outer 

 ectoderm and inner endoderm. Most of the representatives do not 

 develop skeletal structures, but coral polyps produce hard, cal- 

 careous cases around themselves. In several species there is the 

 typical alternation of generations of attached and free-living forms. 

 Most coelenterates are attached or very sedentary for at least a 

 part of the life span. 



The radial symmetry is correlated with an attached habit of life. 

 A good many of the attached forms look much like plants and were 

 so described for a long time. 



The digestive process is principally extracellular, being accom- 

 plished by enzymes which are secreted by special cells of the endoderm 

 into the internal or gastrovascular cavity. A limited amount of the 

 digestion, however, takes place within the endoderm cells after par- 

 ticles of partially digested food have been engulfed by these cells. 

 This is called intracellular digestion. Excretion and respiration are 

 carried on by the general surfaces of the body. Asexual reproduc- 

 tion is accomplished by budding and fission. Sexual reproduction, 

 involving production of ova and spermatozoa and their union in 

 fertilization, occurs here too. 



The group is considered among the simplest of metazoans and 

 shows, in a simple way, typical features of this great division of 

 the animal kingdom. Hydra will be studied in detail, because it is 

 readily available, easily collected and handled, and is representative 



303 



