THE COELENTERATES AND CTENOPHORES 91 



nettling cells acquired their unusual location when coelenterates 

 were used as food. 



Class Hydrozoa 



The Hydrozoa are coelenterates usually having an alternation 

 of generations of which the hydroid is frequently the more 

 conspicuous. The hydropolyp lacks longitudinal folds of the 

 entoderm and the mouth opens directly into the coelenteric 

 cavity. The hydromedusa, when present, has a smooth bell 

 margin; the concave surface is partially enclosed by a membrane, 

 the velum, which extends inward from the edge of the bell. 

 In the members of this class the gonads may be borne either on 

 the radial canals or on the manubrium. The germ cells are 

 expelled from the gonads directly through the surface of the body. 



Life History. — The life cycle of Obelia serves well to illustrate 

 metagenesis as it occurs in the Hydrozoa. The hydroid colony 

 starts as a single polyp which, by continued budding, produces a 

 series of individuals alternating along a common stalk. Such a 

 colony is attached to some object by means of a rootlike structure 

 termed the hydrorhiza. In a young colony, all of the individuals 

 are alike and all function as vegetative zooids. Starting near 

 the base of the colony, reproductive individuals begin to make 

 their appearance. These are sac- or vase-shaped zooids, termed 

 gonangia, budded off in the axils between the vegetative indi- 

 viduals and the common stalk. Each gonangium is composed 

 of a central core upon which buds are formed. As development 

 proceeds, these buds display medusan characters and upon 

 reaching full development become detached and emerge from 

 the open end of the gonotheca as free-swimming jellyfish. The 

 medusae produce germ cells which, upon fertilization, undergo 

 cleavage to form a ciliated larva, termed a planula, but little 

 higher in organization than the gastrula. After a short period 

 of free existence the planula becomes attached to some object 

 and transforms into a hydroid through the development of a 

 circle of tentacles and of a hydrorhiza. From this single zooid, 

 an entire colony is produced through repeated budding. 



While budding is the usual method of reproduction during the 

 asexual generation of Hydrozoa, it occurs but rarely in the medu- 

 soid generation. Sarsia and a few other genera are peculiar in 

 that young jellyfishes are budded from the manubrium or from 

 the margin of the bell at the ends of radial canals. 



