POLVZOA 183 
to some very remarkable structures known as_ statoblasts 
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 statoblasts. 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 statoblasts 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- 
gula fluviatilis, 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 
statoblasts, and the winter eggs of Rotifers 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 EnrTo- 
procTa and (0) the Ecroprocra,—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 
