228 THE STUDY OF ANIMAL LIFE CHAP, xn 



which embryos develop which grow into hydroids. This 

 is known as alternation of generations, and is a remarkable 

 illustration of activity and passivity combined in one 

 life-cycle. It may be defined as the alternate occurrence 

 in one life-history of two (or more) different forms 

 differently produced. (See fig. 100.) 



But all the miniature jellyfish in the sea are not the 

 liberated reproductive buds of hydroid colonies. Some 

 w r hich are in structure exceedingly like the liberated 



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medusoids never have any connection with a hydroid. 

 Their embryos grow into medusoids like the parents. 

 Quite distinct from these medusoids, though sometimes 

 superficially like them, are the true jelly fishes which are 

 frequently stranded in great numbers on the beach. 

 These medusae belong to a different series, and some of 

 their features link them rather to the sea-anemones than 

 to the hydroids. 



The sea-anemones and the corals are tubular animals 

 whose mouths are encircled by tentacles, but they are 

 more complicated internally than the polyps of the 

 hydroid type. For the latter are simple tubes, while the 

 sea-anemones and their relatives have a turned-in gullet, 

 and the inside tube thus formed is connected with the 

 outer wall of the body by many radiating partitions, 

 some idea of which can be gained by looking at the 

 skeletons or shells of many corals. Related to the 

 sea-anemones but different in some details, are many 

 colonies, of which Dead-men's-fingers (Alcyonium digi- 

 tatum), the Precious Coral (Coralliumrvhrum), the Organ- 

 pipe Coral (Tubipora musica), the sea-fans or Gorgonians, 

 and the sea-pens or Pennatulids are well-known types. 

 The term " coral," it may be explained, has a wide 

 significance ; it is applied to any ccelenterate with a 

 strongly developed hard skeleton. 



Besides the types of Ccelentera which we have men- 

 tioned there are others, especially certain corals belonging 

 to the hydroid series and known as Millepores, also the 

 Portuguese Man-of-war and its relatives (Siphonophora), 

 which are colonies of more or less mcdusoid individuals 

 with much division of labour, the curious Black Corals 



