xvin STAGES IN EVOLUTION OF PHYLACTOLAEMATA 503 



In Plumatella, the lophophore has become horse-shoe-shaped, 

 and the tentacles are more numerous (38-60). In general form 

 and in the arrangement of 

 the septa this genus resem- 

 bles Fredericella, with which 

 it may easily be confused. 



In Cristatella we have 

 the most highly modified 

 of all the Phylactolaemata. 

 The individuality of the 

 zooecium is here subordi- 

 nated to that of the colony 

 as a whole. The branched 

 arrangement of the zooecia 

 is greatly obscured. The 

 body-cavities have become 



completely confluent, al- FIG. 251. Statoblasts of Phylactolaemata. A, 



though rudiments of the S?^!*WM3i5?5E 



Septa Still exist. The ectO- lintu Pall., x 28 ; D, Cristatella mucedo 



cyst has been lost, with the ;' pe ii u 2 ) ' (A> * " Allman ' B " D> ^ 

 exception of the basal layer 



of the colony. The tentacles are more numerous (80-90); and 

 in accordance with the increase in the elaboration of the genus, 

 its statoblasts belong to the most complicated type known. 



The production of floating statoblasts may seem a strange 

 adaptation to the conditions of fresh-water life, since it might be 

 assumed, a priori, that these structures would be specially liable 

 to be frozen during the winter. The following experiments made 

 by Braem l show, however, that the germinating power of the 

 statoblasts is improved by a certain amount of frost. A 

 number of statoblasts were taken ; half of these were placed in 

 water, which was then frozen ; and these were found to germinate 

 readily when afterwards exposed to suitable conditions. The 

 other half were not subjected to the action of frost ; and these 

 could not be made to germinate, even although the water had 

 been cooled to a point slightly above the freezing point. It thus 

 appears that the buoyancy, so far from being a risk, is a means of 

 exposing the statoblast to the conditions which are most favour- 

 able to its later development. 



1 r. tit. p. 83. 



