CLASS OF THE COEALS, OE POLYPI. 485 



there ramifying, so as to give a resemblance to a vascular 

 system. 



The family of this class which is best known is that of the 

 medusae, amongst which are the rhizostomes (rhizostomatidae), 

 which abound on the coast, and which are remarkable for the 

 singular disposition of the digestive apparatus, the stomach 

 communicating externally by a great number of small canals, 

 terminated by pores at the free extremity of- the tentacles. 

 In this class are included the beroes (of the class ciliograda) 

 which resemble small balloons ; the cestidse, which have the 

 form of a long gelatinous ribbon; and the physophoridee, 

 which have the appearance of a garland of flowers and fruits.* 



The medusae produce eggs like most animated beings, but 

 the young which spring from these in no shape resemble the 

 mother ; they are small ovoid bodies, having their surface 

 provided with vibratile cilia, and which soon are fixed, and 

 as they become developed form zoophytes, already known to 

 naturalists by the name of hydraid polypi (sertularidae, for 

 example) ; these multiply by granulations, so as to constitute 

 colonies of aggregated animals; and the different individuals 

 of the new generation thus produced become free as they are 

 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 ge- 

 neration, has been called metagenesis, or alternating gene- 

 ration. 



CLASS OF THE COEALS OE 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 

 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. 

 470.) 



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 



* In this family, including the physaledae, the body is floated by air celh, 

 and locomotion performed by parts exposed to the wind. R. K. 



