ALTERNATE GENERATIONS. _ 265 
vertical 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 Discophore. 
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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