294 



FRESH-WATER BIOLOGY 



187 (190) Stalk retractile 188 



1 88 (189) 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 height 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 Amphileptus meleagris. Length of zooids 

 50 M. 



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; 

 macn. macronucleus. X 300. (After Kent.) 



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

 but central muscle continuous, causing all of the zooids to 



contract together Zoethamnium Stein. 



Representative species. . . . Zoethamnium adamsi Stokes 1885. 



Bodies about twice as long as broad, tapering to the 

 pedicel; finely striated transversely. Length of zooids 

 60 M- Reported from Niagara River. Attached to 

 algae. 



FIG. 543. Zoethamnium adamsi. cv, contractile vacuole; macn, 

 macronucleus. X 100. (After Stokes.) 



190 (187) Stalk not retractile , . 191 



