CHAP, xvin FRESH-WATER POLYZOA PHYLACTOLAEMATA 493 



The Entoprocta, as we have seen, are represented in fresh water 

 by the genus Urnatella. 



The Phylactolaemata are an exclusively fresh-water group, 

 and they are believed by Kraepelin J to have been derived from the 

 Ctenostomata. Many of their special peculiarities can, with great 

 probability, be regarded as adaptations to a fresh-water existence. 

 This is particularly clear in the all but universal habit of dying 

 down in the winter, and in the occurrence of the so-called 

 statoblasts (Fig. 251), which are hard -shelled reproductive 

 bodies, absolutely restricted to the Phylactolaemata, and capable 

 of resisting the winter's cold and even a certain amount of 

 drying up. Phylactolaemata have indeed been recorded from 

 the tropics ; but it is not yet sufficiently clear how they there 

 behave in these respects. F. Miiller 2 has found these animals in 

 Brazil, where they are said to be more common at certain periods 

 of the year than at others. Stuhlmann has found them in 

 Tropical Africa (Victoria Nyanza, etc.); 3 and Meissner 4 has 

 discov^ed the sessile statoblasts of Plumatella on the shells 

 pres red in the Berlin Museum, of species of the Mollusc 

 Aetheria from various localities in Africa. Fresh-water repre- 

 sentatives of a considerable number of other groups of animals 

 agree with the Phylactolaemata in the possession of reproductive 

 bodies which are protected by hard coats. Such, for instance, 

 are the ephippian ova of Daphnia bodies which have an extra- 

 ordinary external similarity to statoblasts the gemmules of 

 Spongillidae, the winter-eggs of Ehabdocoels and Eotifers, and 

 the cysts of Protozoa. The evolution of these bodies in so many 

 widely different cases may have been due to the selection of 

 variations calculated to minimise the dangers attendant on the 

 drying up of the water in summer, or on its freezing in winter. 



The Phylactolaemata are by no means uncommon, although 

 they can seldom be found without a careful search. Their pres- 

 ence may often be detected by taking advantage of the property 

 of the free statoblasts of rising to the top of the water, where they 

 can be discovered by skimming the surface with a fine hand-net. 



The colonies themselves are usually found attached to water- 

 plants, roots of trees or stones. Most of them flourish best in 



1 T. cit., p. 167. 2 Quoted by Kraepelin, t. cit., p. 83. 



3 Kraepelin, Abh. Vcr. Hamburg, xii. 1893, No. 2, p. 65. 



4 Zool. Aiiz., xvi. 1893 (1894), p. 385. 



