OBSERVATIONS OX FRESH-WATEK POLYZOA. 95 



All fresh-water Polyzoa are annuals, tlie vegetative and the reproduc- 

 tive portions undergoing entire decomposition every year, while in 

 marine forms, several generations of the vegetative portion, i.e., the 

 polypides, form and decompose themselves on the perennial cystid, like 

 leaves on thelmniches of a. tree. This singular phenomenon led many 

 naturalists (Allmnn nud others) to regard the polypide nnd the cystid 

 as two distinct individuals. In the present species also, the duration 

 of vitality of the two portions is by no means the same. The poly- 

 j)ides invarial)ly die after a certain period of existence, usually after 

 the formatioii of younger polyzooids of the fourth or the fifth order, 

 but the cystids remain until the colony itself disintegrates in winter. 

 In the central porticin of a large colony, therefore, we often see only 

 hare cystids, each with o dark grayish mass, the remains of the dead 

 polypide, banging in its cavitv, ;nid yet with statoblasts continuing 

 their development in tiie funiculus. 



About the application of the terms " antei-ior,'' "posterior," 

 "dorsal," "ventral," Ac, there is much diversity of opinion. For in- 

 stance Allman calls the free end of the polypide "anterior," and 

 the fixed end "posterior"; while Hyatt, following E. S. Morse, 

 calls the fixed end " anterior," and the free end " posterior." 

 Huxley homologizes Polyzoa with Tunicata, and names that side on 

 wliich the anus opens " neural," and the side opposite to it " haemal," 

 although there exists no heart. Again, if we were to compare 

 this animal witli Phoronis, we should have to call the narrow spare 

 between the mouth and the anus "dorsal," and nil other parts 

 " ventral." In fact, every one mioht «five different sets of names in 

 orienting the animal, according to his conception ( f the homology 

 which exists between Polyzoa and other nnimals in which the an- 

 terior and posterior, or the dorsal and ventral ])oles are universally 

 recognized. In the following pages, I shall call the fixed end 



