286 Animal Biology 



Examples: Bath (commercial) sponges (Fig. 85, L) ; Spongilla, a fresh-water 

 sponge (Figs. 86 and 87). 



Phylum 3. Coelenterata (se len ter -a' ta) (Gr. koilos, hollow; enteron, digestive 

 tract) 



General Characteristics 



This phylum includes a number of frequently unnoticed marine animals and a 

 few fresh-water forms, such as Hydra and a few fresh-water medusae. The ani- 

 mals are multicellular and possess a single, hollow, central gastrovascular cavity 



Nemabocyst-- 



flaaeWam 



Testis 



Sperm 



Entoderm celL 

 with yacmole 



Rseudopodium- 



Ovary 



Ovum 



Ectoderm 



-Tentacle 



Gastrovascular 



cavity 



^::l -Older bud 



Mesoqlea 



Young bud 



M Entoderm 



^. Basal disk 



Fig. 89. — Hydra, a fresh-water coelenterate, in section, much enlarged and some- 

 what diagrammatic. 



(enteron). There is no anus. The body wall is diploblastic, being composed of 

 two layers of cells, the outer ectoderm and the inner entoderm. Between these 

 two is a noncellular layer, the mesoglea. Tentacles are characteristically around 

 the mouth. The tentacles and body wall contain peculiar structures known as 

 nematocysts (stinging cells). Coelenterates possess radial symmetry, there being 

 from four to six antimeres (parts of a radially symmetrical animal). Many of 

 the coelenterates are sedentary, while certain species are sessile for at least part 

 of the time. The life cycle typically involves an alternation of generations (meta- 



