THE SIMPLEST MANY-CELLED ANIMALS 



333 



does, and grow rapidly. Some of them may grow to be 

 several inches in diameter. The structure of a typical 

 medusa should be observed. 



(D) Specimen of a medusa known as Gonionemus may be pur- 

 chased from the laboratory at Woods Hole, Mass. They may be 



a b 



FIG. 102. A colony of hydroids growing on a mussel shell. The erect 

 branches, a, 6, c, d, show stages in development of branches by budding. 

 The entire colony originated from a single egg developed into a free- 

 swimming larva which attached to the shell and produced the colony 

 by budding. (After Schulze.) 



kept in small bottles for convenience in demonstrations, or be placed 

 in water in watch-crystals. Notice: (1) bell-form; (2) tentacles; 

 (3) mouth at end of the central projection, which is in same position 

 as clapper of a bell ; (4) digestive canals forming a conspicuous cross 

 and a ring around the margin of the bell, and connecting with 

 the mouth as may be seen in a specimen into whose mouth some 

 pigment in water has been injected with a pipette; (5) irregular 



