334 APPLIED BIOLOGY 



bodies (ovaries or spermaries) are attached inside the bell and just 

 beneath the digestive canals ; (6) a muscular membrane around the 

 inner margin of the bell is responsible for the locomotor movements 

 of the medusa through the water. See Figure 100, c. 



A medusa is more highly specialized than a polyp of a 

 hydroid colony, for in addition to the nutritive functions of 

 the polyp, the medusa has a muscular organ for swimming, a 

 ring of nerve-cells with their fibers, and the reproductive 

 organs. 



Embryos. Egg-cells and sperm-cells produced by medusae 

 result in fertilized eggs, which develop into free-swimming 

 embryos. Each embryo after a time settles down on rocks, 

 piles, and other objects, and becomes a hydra-like polyp 

 which looks much like one of the ordinary polyps seen in 

 colonies. This grows and forms buds, and by repeated bud- 

 ding a new hydroid colony is formed. 



289. Alternation of Generations. The above account 

 makes it evident that the hydroid colony and the medusae 

 alternate just as do the fern plant and the prothallium ( 226). 

 The hydroid colony forms ordinary polyps and medusae 

 by budding (an asexual reproduction) ; the medusae produce 

 germ-cells (ova and sperm-cells), which start new hydroids 

 able to bud and form colonies. The colony is' the asexual 

 generation, the medusa is the sexual generation. In very 

 many species of hydroids this alternation of generations 

 is just as necessary a part of the life-cycle as it is in ferns and 

 mosses. Medusae of such species cannot produce eggs 

 which are able to develop into medusae without the hydroid 

 colony stage; and conversely, the hydroid colony cannot 

 form new colonies without the medusa stage. 



There are some species of medusae closely related to 

 those formed by hydroids whose eggs develop directly into 

 new medusae ; that is, these species do not have alternation 

 of generations. 



In one of the common hydroids of the New England coast 



