CCELENTERATA — HYDROZOA 



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opening, — the mouth. The stems, an inch to an inch and a half 

 long, simple or branched, spring at intervals, usually in clusters, 

 from a creeping and much branching root-stock (hydrorhiza). 

 This species is very abundant off the northeast coasts of North 

 America from New Jersey to the Arctic, also on the northwest 

 coasts of Europe. 

 It occurs in espe- 

 cial profusion be- 

 tween tides at- 

 tached to the 

 rock weed, Fucus. 



Each polyp is 

 continuous at its 

 base with the 

 central canal ; its 

 entire surface se- 

 cretes a protec- 

 tive layer of 

 chitin composed 

 of superposed 

 lamellae. When 

 mature it largely 

 draws away from 

 this protective 

 covering. There 

 is thus formed a 

 cup out of which 

 the animal can 

 expand to get 

 food, and into 

 which it can 

 withdraw for 

 protection. 



Its food consists of small, usually microscopic animals and 

 plants, as well as of organic fragments. These it catches with 



Fig. 39. — A portion of a colony of the hydroid, Sertidaria 

 pumila, from the coast of Massachusetts, cce., coenosarc. 

 — the common flesh uniting; the various individuals of 

 the colony, — through which passes the central canal ; 

 dig., digestive cavity; gth., gonotheca, or cup lodging 

 a reproductive polyp; hth., hydrotheca, or cup lodging 

 a feeding polyp; m., mouth; p., a polyp expanded, in 

 feeding position; />'., a polyp contracted, in protected 

 position; /., tentacles. X 60. 



