THE SIMPLEST MANY-CELLED ANIMALS 



335 



the medusae sometimes do not develop enough to become 

 free-swimming, and remain attached to the colony. However, 

 they produce embryos in the usual way, and the embryos 

 swim away and start new colonies. 



290. The Larger Jelly-fishes. The term " jelly-fish " 

 usually applies to some of the large medusae, some of which 

 are from five to eight feet 

 in diameter. Their general 

 structure resembles that of 

 the medusae described above ; 

 but the life-history does not 

 include a hydroid colony. 

 Instead, an egg produced 

 by the large medusae develops 

 into a ciliated embryo, which 

 swims for a time, settles down 

 and changes into a small 

 animal looking like a hydra. 

 This divides transversely into 

 a number of disks so that the 

 animal now looks like a pile 

 of saucers. Each of these 

 saucer-shaped bodies becomes 

 a medusa, and grows rapidly, 

 of generations. The large size of the adults is partly due to 

 absorption of water, more than nine tenths of a jelly-fish 

 being water. See Fig. 103. 



The Ctenophores or comb-jellies, named because of eight 

 bands of comb-like appearance, consisting of vibrating plates 

 which cause locomotion, are the highest ccelenterates. 

 Museum specimens should be examined. The larger ones 

 are often the cause of the phosphorescent masses seen in 

 sea-water when a boat is being paddled on a dark mid-summer 

 night. Ctenophore eggs develop into young ctenophores; 

 there is no alternation of generations. 



FIG. 103. Development of one of 

 the larger species of jelly-fishes. 

 1, larva developed from an egg; 2, 

 3, 4, 5, stages in development of 

 larva into a fixed hydra-like or- 

 ganism; 7, 8, 9, division into many 

 disks, each forming a young me- 

 dusa (11, 12). (From Hatschek.) 



There is, then, alternation 



