ALTERNATE GENERATIONS. 263 
motion, and I will add one word of the peculiar- 
ity in their structure which causes this effect. 
The Ctenophore 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 Ctenophorz: a, anal aperture; 8, radiating tube ; 
c, circular tube; d,e, f,9g,, 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 Ctenophore. 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 
