504 POLYZOA 



Braem supposes that the beneficial action of irost is due 

 to a lowering of the vital energy of the statoblast. As in the 

 case of reproductive bodies known in many other fresh -water 

 organisms, the statoblast germinates only after a period of rest. 

 Although this period is often shortened by a lowering of the 

 temperature, it can also be induced by the exclusion of air, as in 

 an experiment during which the statoblasts were enclosed in air- 

 tight tubes. The respiratory processes were thereby lessened, 

 and the germinating power was materially improved. 



Since the development of the statoblasts depends largely 

 on the temperature, the first warm weather in early spring will 

 probably induce the germination of those which are floating; 

 and the young colony, leaving the protection of the statoblast, will 

 become susceptible to frost. But even if the first-formed colonies 

 are killed off by a subsequent frost, other statoblasts which have 

 remained in the mud during the winter are disentangled from 

 time to time, and germinate on reaching the surface. 



Distribution. 'The protective value of the shell is also 

 shown by the fact that the statoblast may be kept for some 

 months in a dry condition without losing its power of germination. 

 There can be little doubt that the capability of withstanding 

 desiccation enables the species to enlarge its area of distribution. 

 It is asserted that fresh-water Polyzoa decrease in abundance in 

 proportion to the distance from the mouth of the river in which 

 they are found. The current will naturally tend to bring together 

 the statoblasts from the Folyzoa growing in the upper waters. 



Nothing is more surprising than the wide geographical dis- 

 tribution of the rhylactolaemata. The European genera are 

 all recorded from North America. Fredcrieella, Plumatella, and 

 Lophopus are further recorded from Australia ; while Plumatclla 

 is known to occur also in Malacca, the Philippine Islands, India, 

 Japan, Africa, and South America. It is even stated that some of 

 the Australian species are identical with those found in Europe. 



Some of the fresh-water Polyzoa are extremely variable, and 

 observers are by no means agreed in deciding whether certain 

 well-known forms are to be regarded as varieties or as species. 

 While certain genera, such as Cristatclla and Loplwpus, are 

 comparatively constant in their form, Plumatella is excessively 

 variable. Plumatclla has a number of species greater than that of 

 any other form, and the genus lias a wider distribution than any 



