298 H. F. THACHER. 



"disappear" when their condition of growth is disturbed /. e., 

 the former being brought into contact with a solid, the latter 

 being suspended horizontally so that its relation to gravity is 

 changed. . Eudendrium, according to some workers, sheds its 

 hydranths when brought into the laboratory, but I have also 

 often found absorption occurring under the same conditions, 

 and Eudendrium tcnuc, a smaller and more delicate form than 

 Eudendrium racemosum, responds in this way even more con- 

 stantly. Pt'/inaria 1 has recently been examined by Cerfontaine 

 who finds that the day after the hydroids have been collected 

 " ca materialse trourait dans un mauvais etat, les polypes qui 

 persistaient etaient morts, les parties mollas s'etaient retirees 

 dans la perisarque et les extremitees du coenosarque reduit 

 s'etaient cicatricees. Si 1'ou conserve les branches, en mainte- 

 nant une circulation d'eau de mer, ou les voit souvent reprendre 

 de la vigueur. . . . Ou peut de cette fac_on determiner experi- 

 mentalement une repetition de la regeneration spontanee. A la 

 suite des troubles brusques produits dans les conditions d'etre de 

 ces organismes, par la recolte, le transport, le changement d'eau, 

 le changement de temperature, de lumiere, etc., ou determine 

 rapidement la destruction des polypes ; mais bientot, il semple se 

 produire une acclimation rapide, et aussitot une nouvelle regener- 

 ation commence." Tubularia never absorbs its polyps but sheds 

 them soon after being collected, and after a day or so if undis- 

 turbed, new polyps grow out from the old stalk, a new growth 

 of stalk also taking place behind the head. 



It seemed possible that the absorption of the heads of Cani- 

 panularia might be analogous to that in these other forms, in 

 which case it should occur even when, not in contact with solids. 

 To test this, I left the hydroids still growing on bits of wood, and 

 placed them in the dishes, so that they were completely sur- 

 rounded by water. Nevertheless the polyps began to absorb and 

 by the end of twelve hours had almost entirely disappeared, while 

 a few new ones were beginning to form from the old stalks. I 

 also noticed on examining dishes of unused hydroids that had 

 been standing over night, a large percentage of absorbing polyps. 



1 " Recherches experimentale sur la Regeneration et 1'Heteromorphose chez as- 

 troides calycularis et Pennaria Carolinii," Archives de Biologie, XIX., 1902. 



