226 The Study of Animal Life part in 



ways from other Coelenterates, thus the characteristic 

 stinging cells are modified into adhesive cells. 

 The first and second series, separated by differences of structure 

 and development, are yet parallel. In both there are polype-types ; 

 in both medusoid types ; in both there are single individuals and 

 colonies of individuals ; in both there are " corals." We may 

 compare a Hydra with a sea-anemone, a. medusoid with a jelly- 

 fish, a hydroid colony with Dead -men's- fingers, Millepores with 



Fig. 42. — The alternation "ol generations in the common jellyfish Aurelia. i, 

 the free-swimming embryo ; 2, the embryo settled down ; 3, 4, 5, 6, the de- 

 veloping asexual stages, or hydra -tubse; 7, 8, the formation of a pile of 

 individuals by transverse budding ; 9, the liberation of these individuals ; 

 10, II, their progress towards the free-swimming sexual medusa form. (From 

 the Evolution of Sex ; after Haeckel.) 



the commoner reef-corals. Moreover, we may compare a medusoid 

 liberated from a hydroid with Aurelia liberated from its fixed polype- 

 stage, and permanently-free medusoids with jellyfishes like Pelagia. 

 These are physiological parallels. 



The sedentary polypes are somewhat sluggish, with a tendency 

 to bud and to form shells or skeletons of some kind. The free- 

 swimming medusoid types are active, they rarely bud, they do not 

 form skeletons, but their activity is sometimes expressed in 



