CGELENTERATA 



6 7 



Order 2. Hydromedusae 



The Hydromedusae (Gr. vBwp, water, and \leSovaa, Medusa) 

 are marine Hydrozoa and include both polyps and medusae. 

 The polyps are known as hydroids and resemble Hydra de- 

 scribed above except in two respects — the hydroids generally 

 form permanent colonies, and they do not themselves produce 

 germ-cells. The colonies are formed by budding, and the 

 codenteron of each individual is continuous through the cceno- 

 sarc with that of the others (Fig. 46). 



OP~ 



4) 



• 



•b 



IMA 



FIG. 50. Afillipoia akicortiis. A, part of skeleton, natural size; B, portion of 

 surface magnified; C, vertical section, magnified, d.p, dactylopores ; g.p, gas- 

 tropores; tb, tabulae. (After Nicholson and Lydekker, from Parker and Has- 

 well's Text-book.) 



Here and there on the colonies budlike outgrowths occur, 

 which do not develop into polyps, but into medusae, which ulti- 

 mately separate from the hydroid, and swim about as free-living 

 individuals. These medusae produce germ-cells from which in 

 turn polyps develop, and thus an alternation of generations is 

 established. While we may call this process typical for the 

 Hydromedusae, it is by no means constant. For in some 

 hydroids the buds which we might expect to form medusa 

 never attain a complete development, but remain attached and 

 there produce germ-cells, which escape and develop into new 



