70 STRUCTURAL AND SYSTEMATIC ZOOLOGY 



one end to submerged plants, while the free end con- 

 tains the orifice, or mouth, crowned with tentacles, by 

 which the creature feeds and creeps. The body wall 

 consists of two cellular layers ectoderm and endo- 

 derm. These surround a central cavity with one 

 opening. The animal may be compared to a bag with 



a two-layered wall, and 

 with tentacles around 

 the opening. It buds, 

 and also reproduces by 

 eggs. The buds, when 

 adult, become detached 

 from the parent (Figs. 

 17, 1 8). 



In most of the other 

 Hydrozoa the colony 

 is permanent, and sup- 

 ported by a horny 

 skeleton. There are 

 two kinds of Polyps in 

 each colony, one for 

 feeding and the other 



FIG. 17. Hydra: 2, with tentacles fully extended; for reproduction (Fig. 

 3, creeping ; 4, with ingested prey : 5, budding. ~ . - 



20). Sometimes the 



reproductive Polyps are separated from the stock 

 in the form of little jellyfishes, and are then called 

 medusae (Figs. 20 m, 21). Belonging to this class 

 are Hydractinia, found on the shells inhabited by the 

 hermit crab ; the elk-horn coral (Millepora) ; and the 

 beautiful Portuguese man-of-war, consisting of a bladder- 

 like float from the bottom of which depend tentacles 

 many feet in length and several kinds of polyps, the 

 tentacles being covered with stinging cells, which aid 

 in capturing the prey and in defending the colony. 



