POLYZOA 183 



to some very remarkable structures known as statohlasts 

 These have the morphological significance of buds ; they con- 

 sist of a little heap of cells which secrete around them a 

 chitinous shell. 



The freshwater Polyzoa usually die down at the approach 

 of winter, and the continuance of the race is provided for by 

 the persistence of the statohlasts. These structures are usually 

 formed during the autumn, and escape after the decay of the 

 parent organism. Their chitinous shell in most cases is a 

 complicated structure ; part of it contains air vesicles, the air 

 being secreted from the protoplasm of the cells which have 

 formed the shell. Thus the latter forms a float; in some 

 cases, however, the statohlasts are attached to submerged 

 stones, and the floating ring is then rudimentary or absent. 

 The formation of these structures, capable of resisting the 

 winter frosts, in freshwater Polyzoa acquires an additional 

 interest when they are compared with similar contrivances 

 found in other members of the freshwater fauna. Spon- 

 gilla Jluviatilis, the freshwater sponge, also dies down at the 

 approach of winter; it also forms remarkable bodies, termed 

 gemmules, which consist of a collection of cells protected 

 by curious spicules termed' amphidiscs, and from which a new 

 sponge arises in the spring. In the same connection the 

 ephippian eggs of Daphnia, which are also supported by an air 

 float, and in other respects have a striking resemblance to some 

 statohlasts, and the winter eggs of Eotifers and Planarians, may 

 be mentioned. The low temperature of winter, which affects 

 the comparatively small bulk of fresh water much more than 

 the ocean, where beyond a depth of a few fathoms the cold 

 hardly affects the temperature of the water, has apparently 

 called forth these modifications. 



A. ECTOPROCTA. 



The Polyzoa are divided into two groups, — (a) the Ento- 

 PEOCTA and (&) the Ectopeoota, — according as to whether the 

 anus is included or not in the circlet of tentacles which sur- 

 rounds the mouth. The Ectoprocta are further characterised 

 by the possession of a well -developed coelom. This group 



