484 



EVOLUTION OF ANIMALS 



Part V 



Mole 



SEXUAL REPRODUCTION 

 Female 



Fold of lip 



Swimming 

 ephyra 



Sperm 



Strobilo 





"^^ SCyphistoma 



'0 develops 

 he folds 

 the lips 



Swimming 

 iarvQ 



Polyp attached 

 to rock 



Fig. 23.14. The life cycle of the common jellyfish, Aurelia. During their com- 

 plete cycle jellyfishes have different forms and habits. The largest of these are the 

 male and female medusas, 6 to 10 inches across the disks in Aurelia. All the other 

 forms are minute. The embryo is produced by the union of sperm and egg, and 

 sheltered in the streamer-like lips of the parent. The larva swims by cilia and trans- 

 forms into a hydra-like polyp. In the following stages, scyphistoma and strobila, 

 the animal divides into a series of saucer-shaped young ones. Finally these separate 

 and as ephyras, developing males or females, they swim free. 



heavily loaded with more stinging cells. Within a few hours, they are reduced 

 to broth by secretions strong enough even to digest chitin. Particles of food 

 are engulfed by nutritive cells and digestion is completed within them as it is in 

 the similar cells of hydra. 



Jellyfishes have a very definite sense of balance. If one of them is tilted out 

 of horizontal position it will contract more strongly on the upper than on the 

 lower side and bring itself back to a horizontal position. If the organs of 

 balance in the notches are all removed from one side and that side is upturned 

 as before, the animal will not attempt to right itself. The ovaries and testes, 

 always borne on separate individuals, are the four horse-shoe-shaped bodies 

 in the floor of the central enteron, the most conspicuous structures in the 

 animals. 



The embryo goes through its early development within the folded lips of the 

 female, becomes a ciliated free-swimming larva, and then a polyp that settles 

 upon a rock or seaweed. There it may grow for months budding off one young 



