ALTERNATE GENERATIONS. 265 



rertical with horizontal tubes and the external 

 appendages accompanying them. 



But, whatever their differences may be, a very 

 slight effort of the imagination only is needed to 

 transform any one of these forms into any other. 

 Reverse the position of any simple Hydra, so that 

 the tentacles hang down from the margin, and 

 let four tubes radiate from the central cavity to 

 the periphery, and we have the lowest form of 

 Jelly-Fish. Expand the cup of the Hydra to 

 form a gelatinous disk, increase the number of 

 tubes, complicate their ramifications, let eyes 

 be developed along the margin, add some exter- 

 nal appendages, and we have the Discophorc. 

 Elongate the disk in order to give the body an 

 oval form, diminish the number of main tubes, 

 and let them give off vertical as well as horizontal 

 branches, and we have the Ctenophore. 



In the Class of Polyps there are but two Or- 

 ders, — the Actinoids and the Halcyonoids ; and 

 I have already said so much of the structure of 

 Polyps that I think I need not repeat my remarks 

 here in order to show the relation between these 

 groups. The body of all Polyps consists of a 

 sac divided into chambers by vertical partitions, 

 and having a wreath of hollow tentacles around 

 the summit, each one of which opens into one of 

 the chambers. The greater complication of these 

 parts and their limitation in definite numbers 



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