Survey of the Animal Kingdom 289 



genesis) between the hydroid (hydralike) and medusoid (medusa-like) stages. 

 Not all coelenterates have metagenesis. Obelia illustrates a type which has meta- 

 genesis by having both a hydroid and medusoid stage in its life cycle (Fig. 88). 

 Number of species of Coelenterata, 9,500. 



Classification of the Phylum Coelenterata 



Class 1 — Hydrozoa (hidro-zo'a) (Or. hydra, water; zoon, animal). — 

 These coelenterates possess a gastrovascular cavity which is not held in position 

 by membranous mesenteries. They do not possess a true gullet or stomodaeum 

 as do the Anthozoa. Sex cells are discharged directly to the exterior. Certain 

 species have alternation of generations (metagenesis), while others do not. When 

 a medusa is formed in the life cycle, it has a velum (membrane on the under 

 surface). 



jr.- ■ ■■it :.;r'-*^ -^ •■-■■»■:•- *r-:-.>^S** ■ -vS. '3 



Tloat h]adder 



Polyps -..-^ 



Jentade-- 



Hg. 93. — Portuguese man-of-war {Physalia sp.) of the phylum Coelenterata, 

 class Hydrozoa. This colonial coelenterate floats on the surface of the sea. Male 

 and female zooids, vegetative polype, and long tentacles with nematocysts are 

 suspended from the float bladder. 



Examples: Hydra (Figs. 89, 90), Obelia (Fig. 88), many of the smaller 

 jellyfishes (medusae) , Gonionemus (Fig. 92), and Portuguese man-of-war (Fig. 

 93). 



Class 2 — Scyphozoa (sifo-zo'a) (Or. skuphos, cup; zoon, animal). — These 

 types have their gastrovascular cavity held in position by membranous mesenteries. 



