950 FRESH-WATER BIOLOGY 



at a depth of 23 to 36 meters, Paludicella ehrenhergii and Frederi- 

 cella sultana were abundant. Although Cristatella is usually found 

 on the underside of floating Hly pads or in other situations near 

 the surface, I have obtained it from the still waters of Trinity 

 Lake, Westchester County, New York, at a depth of 2 to 3 meters. 

 Asper records dredging Fredericella sultana in certain Swiss lakes 

 at a depth of 50 to 80 meters. Little light penetrates to such a 

 depth, and we may conclude that hght is not at all directly neces- 

 sary for the development of fresh-water Bryozoa. Indeed, masses 

 of Paludicella are sometimes obtained from water pipes where they 

 flourish to an alarming extent. 



The Bryozoa have become adapted to Hfe in ponds by forming 

 statoblasts at certain seasons of the year. The entire significance 

 of the statoblasts has not been determined. Typically, they winter 

 over and one may find the shores strewn with them in the early 

 spring. They hatch out in New England late in May or early in 

 June. So the statoblasts have come to be regarded as winter buds, 

 or adaptations to preserve the race from being killed ofT by freezing 

 of the water. They often begin to develop early in the summer, 

 and I have observed what has been seen by European observers, 

 that some statoblasts hatch in nature even in September. Also 

 Fr. Miiller has informed Kraepehn that the fresh-water Bryozoa 

 of Blumenbau, Brazil, which experience no winters, nevertheless 

 form statoblasts. It seems fair to conclude that there are other 

 functions performed by the statoblasts than resistance to winter. 

 For instance, they serve to maintain the species during drought, 

 or they aid in distribution by clinging to the waterfowl or resisting 

 the action of digestive fluids. The wide distribution of the species 

 of fresh-water Bryozoa indicates the value of the statoblast in the 

 process of dispersion. For a detailed account of the distribution 

 of the fresh-water species in the United States see Davenport (1904). 



Preserving. — The chief difficulties in the way of preserving 

 fresh-water Bryozoa arise, first, from the rapid contraction of the 

 polypides into the corm, and, secondly, in the case of the gelatin- 

 ous forms, from the large amount of water in the body; for, if the 

 specific gravity of the killing or preserving medium is very differ- 

 ent from that of the water, distortion will occur. 



