ALTERNATE GENERATIONS. 



263 



motion, and I will add one word of the peculiar- 

 ity in their structure which causes this effect. 

 The CtenophoraD differ from the Jelly-Fishes de- 

 scribed above in sending off from the main cavity 

 only two main tubes, instead of four like the 

 others ; but each of these tubes divides and sub- 

 divides in four branches as it approaches the pe- 

 riphery. From the eight branches produced in 

 this way there arise vertical tubes extending in 



Idyia roseola ; one of our Ctenophorae : a, anal aperture 5 &, radiating tube ; 

 c, circular tube 5 d, e,f, g, fi, rows of locomotive fringes. 



opposite directions up and down the sides of 

 the body. Along these vertical tubes run the 

 rows of little locomotive oars, or combs, as they 

 have been called, from which these animals derive 

 their name of Ctenophorae. The rapid motion 

 of these flappers causes the decomposition of the 

 rays of light along the surface of the body, pro- 

 ducing the most striking prismatic effect ; and it 

 is no exaggeration to say that no jewel is brighter 



