CCELENTERATA 



6 1 



which may serve for food. Somewhat similar structures have 

 already been described as occurring in some of the Ciliata. 



In the Cnidaria we find two kinds of individuals very different 

 in appearance, but fundamentally not so unlike one another. 

 The one is attached, 

 and the general shape 

 of the body is that 

 of a hollow cylinder, 

 closed at the attached 

 end, with the mouth 

 at the opposite end, 

 which is surrounded 

 by one or more rings 

 of slender, fingerlike 

 processes known as 

 tentacles. Such in- 

 dividuals are called 

 polyps. The other 

 kind is free-swimming 

 and disc- or bell- 

 shaped, with tentacles 

 around the margin of 

 the bell ; these are 

 known as medusae 

 (Fig. 46). Some of 

 the Cnidaria live in 

 fresh water but by 

 far the greater num- 

 ber are marine, and 

 include such animals 

 as are known popu- 

 larly as the Portuguese man-of-war, sea-anemones, jellyfishes, 

 and corals. 



CLASS I. HYDROZOA 



The Hydrozoa (Gr. vSwp, water, and %wov, animal) are found 

 in fresh and in salt water, and include both polyps and medusa-. 

 The polyps conform to the typical Cnidarian described above. 

 The ectoderm and entoderm are separated by 3 basement 



Fig. 44. Euplectella, skeleton, siliceous. (Photographed 

 from specimen, by the author.) 



