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TEXT-BOOK OF ZOOLOGY 



produced by budding remain in connection with each other. In this 

 manner, by continuous budding, polyp colonies, or polyp stocks, are 

 formed, which resemble small trees or shrubs, delicate cushions of moss, 

 etc. (hence old name " zoophytes," i.e., plant animals). All the members 

 of a polyp colony are, however, united by a canal, which traverses the 

 stem and branches of the stock, so that the food captured by one of the 

 polypites benefits also all the other "persons " of the colony. Such a 

 colony, with its frequently numerous ramifications, of course requires 

 special supporting apparatus for maintaining its separate parts erect in 



DEVELOPMENT OF AUUELIA AUKITA. 



1, Free swimming larva (planula) ; 2, younger, and 3, older, polyp (or hydratuba) stage, which 

 in 4 is divided into a number of discs (strobila) ; 5, separated uppermost disc, i.e., a young 

 medusa (Ephyra) which develops into the adult form 6 (1 to 5 magnified) ; 6, a young 

 jelly-fish. 



the water. (Compare with colony-building corals and with solitary 

 polyps.) Accordingly, the body surface secretes a chitinous envelope 

 (polypary; see p. 308), in the form of a tube, which encloses the stem 

 and branches of the colony, and usually also the separate polyps. The 

 coloured chitinous covering of a polyp stock (Sertularia), which extends 

 over large areas of ground in the shallow waters of the North Sea, has 

 recently been introduced into commerce under the name of sea-moss, 

 and is used for artificial flowers. 



In every colony we meet with a remarkable division of labour. In 

 addition to the animals which are engaged in nutritive functions, 



