Unicellular and Multicellular Animals 



general appearance from the Hydra or sea-anemone, yet the one 

 form may be somewhat easily derived from the other ; we have 

 only to imagine that a Hydra is turned upside down, that it is 

 squashed, vertically, until the internal cavity is greatly reduced, 

 and the circumference, especially in the region of the tentacles, 

 greatly increased, and we should have something resembling a 

 Medusa. That the two types are actually closely related is 

 shown by the fact that there is in the life-history of one group 



^man 



Fig. 35. — Diagram of Medusa. 

 7^ad^ Radial canals, with reproductive bodies, ; r, ring canal ; /, tentacle canal. 



of Coelenterates a regular alternation between the one and the 

 other. 



If the above general conception of the structure of the Medusa 

 be borne in mind, its details will be easily understood. The 

 internal cavity, instead of being simple, has become complicated, 

 through the obliteration of certain parts of it, where the upper 

 and lower walls come in contact. What is left is a comparatively 

 small cavity immediately above the mouth, a number of sym- 

 metrically arranged canals radiating out from this, and a ring 



canal connecting the ends of these with each other. Another 

 D 49 



