294 



FRESH-WATER BIOLOGY 



187 (190) Stalk retractile. 



1S8 



(i? 



Zooids contracting independently. Bodies bell-shaped. Central 

 muscle interrupted at the union of the stalk and the branch. 

 Ciliated spiral forming about one and a half circles. Nu- 

 cleus horseshoe-shaped. . . . Carchesium Ehrenberg. 



Representative species. Carchesium polypinum Kent 1882. 



Colonies often reaching a lieight of one-eighth of an inch. At- 

 tached to the under surfaces of stones or floating sticks in fresh- 

 water pools or running streams. The colony may be the 

 temporary host of Ampkileptus meleagris. Length of zooids 



S° f- . 



Some interesting work has been done on the nucleus of this 

 species by Miss M. Greenwood. (The Journal of Physiology, 

 Vol. XX, pp. 427-454.) It was found that the normal activity 

 causes a drain on the organism which, if not offset by sufficient 

 repair due to the lack of nutrition, results in the more fluid char- 

 acter of the macrosomes of the nucleus. 



Fig. 542. Carchesium polypinum. Terminal branch with two zooids; 

 tnactif macronucleus. X 500. (After Kent.) 



189 (188) Zooids contracting together. Bodies very similar to Carchesium 



but central muscle continuous, causing all of the zooids to 



contract together. . . Zoothamnium Stein. 



Representative species. . . . Zoothamnium adamsi Stokes 1885. 



Bodies about twice as long as broad, tapering to the 

 pedicel; finely striated transversely. Length of zooids 

 60 II. Reported from Niagara River. Attached to 

 algae. 



Fig. S43. Zoothamnium adamsi. cv, contractile vacuole; macn 

 macronucleus. X lOO. (After Stokes.) 



190 (187) Stalk not retractile 191 



