530 ZOOLOGY. 



developed, and metamorphosed into medusae. This succession 

 of individuals of two kinds, which alternately succeed each 

 other and present the same forms only at the second 

 generation, has been called metagenesis, or alternating gene- 

 ration. 



CLASS OF THE COEALS, OK POLYPI PEOPEELY SO CALLED. 



619. Some confound under the name of polypi the 

 bryozoaria, of which we have already spoken in treating of the 

 molluscoids ( 615), and the corals, or polyps properly so 

 called, which have a structure entirely different and much 



Fig. 556. Polyp of the genus Astroides; a Coral. 



less complete. These are animals with a cylindrical body, 

 soft, and pierced at one extremity by a central mouth, sur- 

 rounded with tentacles, and without vibratile cilia (as Fig. 

 556). 



This orifice holds the place of anus, and leads directly, or 

 by the intermedium of a membranous tube, into a large 

 cavity occupying all the body, extending superiorly into the 

 tentacles, and lodging the ovaria suspended to its walls. The 

 inferior extremity of the polyp is contracted, so as to adhere 

 to foreign bodies, on which the animal is destined to live 

 fixed to them ; its skin generally hardens to a large extent, 

 so as to form a horny or calcareous envelope, analogous to 



