96 



MEDUSA POLYPI. 



Fig. 86. MEDUSA. 



a their internal organs consist almost 

 exclusively of a stomach, hollowed 

 in the substance of the body, from 

 which arise different branched canals. 

 The Medusa belong to this class. 

 The body is broad, and more or less 

 convex, resembling a disk or the cap 

 of a mushroom (Jig. 86, a). The 

 margin and centre of the cap are 

 furnished with tentacles (6), which 

 probably serve them to seize small 

 mollusks or zoophytes, and convey 

 them to the mouth. They swim by 

 slowly contracting the margin of the 

 cap, and thus expelling the water 

 contained in its concavity ; they are 

 seldom seen on the surface except 

 in calm weather. Many of these 

 animals contribute to the phos- 

 phorescence of the sea, diffusing a whitish light. 



16. CLASS OF POLYPI. Under the name of 

 polypi is included a great number of animals, 

 possessing a cylindrical or oval body, with an 

 opening at one of 

 its extremities, sur- 

 rounded by long 

 tentacles (Jig. 87). 

 The structure of 

 polypi is very sim- 

 ple, and their facul- 

 ties very limited. 

 Most of them live 



fixed to other bodies, by the posterior 

 extremity, and all their movement 

 consists in extending and contracting 

 their tentacles, and drawing the an- 

 terior portion of the body into itself. 

 They are multiplied in two ways : 

 sometimes they produce eggs, which 

 detach themselves, and are expelled, 

 and their development is left to chance; 

 at other times, buds spring from the 

 surface of the body, which never 

 separate, but become so many new 



Fig. 87. ACTINIA. 



Fig. 88. SERTULARIA. 



16. What are polypi ? What are their characters? 



