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TEXT-BOOK OF ZOOLOGY 



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

 manner, by continuous budding, ]><>hjp 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 Aukelia Ai/hita. 



1, Free swimming larva (planula) ; 2, younger, and 3, older, polyp (or hydratulia) 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 {Scrtularia), 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, 



