MOSS ANIMALCULES (BRYOZOA) 949 



are then set free, and, after a time, settle and affix themselves. 

 Like other sessile forms, Bryozoa have gained a variety of methods 

 of reproduction. Ordinary sexual reproduction and budding have 

 been already mentioned. In some Phylactolaemata Pectinatetta 

 and Cristatella the colonies occasionally undergo fission and 

 move apart, and the same process occurs regularly in Urnatella. 

 Statoblast formation, which occurs on the funiculus in Phylacto- 

 laemata, is mentioned below. 



The fresh- water Bryozoa do not constitute a natural group of 

 animals, but have descended from ancestors belonging to widely 

 distinct families. There can be no question that these ancestors 

 were marine animals. Excepting the suborder Phylactolaemata, 

 all fresh- water Bryozoa belong to groups most of whose representa- 

 tives are marine. The fresh- water forms seem to have made their 

 way up estuaries and rivers to lakes and ponds. Here they acquired 

 the capacity of forming statoblasts or hibernacula, by virtue of 

 which the species was enabled, on the one hand, to survive the 

 winter and, on the other, to be carried by waterfowl and winds 

 over divides from one drainage basin to another. Thus the fresh- 

 water species have become nearly cosmopolitan. Plumatella 

 princeps has been found in North and South America, through- 

 out Europe, in Molucca, Japan, and Australia i.e., in all but one 

 of the great geographical divisions of the land areas of the globe. 



The fresh- water Bryozoa live in all kinds of fresh waters and are 

 indeed among the most ubiquitous of aquatic animals. They are 

 found in stagnant pools and in rushing rivers, although particular 

 species favor special habitats. The different species of Pluma- 

 tella occur in varied habitats. Paludicella and Pectinatella favor 

 running water and Lophopus, Cristatella, and Plumatella polymorpha 

 favor quiet ponds. The fresh- water Bryozoa feed on microscopic 

 organisms which are caught in the vortex created by their ciliated 

 tentacles. Diatoms are especially conspicuous objects in their 

 alimentary tracts. Since diatoms require light for their construc- 

 tive metabolism, they are found chiefly in the upper strata of the 

 water, and consequently Bryozoa are usually not found at great 

 depths. However, in a mass of material dredged by Professor H. 

 B. Ward from the Middle Ground, Traverse Bay, Lake Michigan, 



