THE PHYLA COELENTERATA AND CTENOPHORA 



199 



f 



Yoxxxig 



Mai; lire 





"^ 



Male, 



nt 



F^'inal* 



, . ,^\XNN\N\\NNNNV^V^-SN'^\NNNVXNXXNNNNN\\NNNNNNNNV\V.NV<XV 



Figure 10.18. Reproduction in Hydra. Sperm are shed from the testes of the male, 

 and swim to the females where they fertilize the mature eggs. 



Stage while still attached to the parent. The plantila lacks cilia, and 

 secretes a surrounding shell which later falls off. It hatches later as a 

 young polyp. 



The polyp feeds in typical fashion on small aquatic animals. It is 

 no more complex than the polyp of Gonionemus, except that it is con- 

 siderably larger and has four kinds of nematocysts. One kind is radically 

 different (Fig. 10.17, right), having no spines, poison or opening at the 

 tip of the thread. Instead the thread coils tightly after eversion, often 

 encircling minute spines or hairs on the prey and holding it fast. The 

 structural simplicity of hydra has suggested to some investigators that it 

 is a juvenile form that becomes sexually mature without metamorphosis. 



Asexual buds do not become frustules but develop mouths and 

 tentacles while still attached to the parent. Later the base constricts and 

 the offspring creeps away. A parent may have several such buds and 

 temporarily resemble a colonial hydrozoan (Fig. 10.17, left). 



Unlike most hydrozoan polyps, which are permanently attached to 

 the bottom, hydra moves from time to time. The polyp may slide slowly 

 on its base at the rate of a few inches a day, or it may somersault at a 

 more rapid rate by alternately attaching tentacles and base. 



70. The Phylum Ctenophora 



Comb jellies have a spherical or vertically elongate body plan in 

 contrast to the umbrella shape of medusae. Familiar representatives are 



