INTRODUCTION. Vll 



the alimentary zooid. But the structural type which it 

 represents appears under various modifications, subser- 

 vient to very different functions, as we shall see here- 

 after. At present we are to regard it, in its primitive 

 form, as the house of the polypide. 



Its walls are made up (with, perhaps, a single excep« 

 tion^) of two membranes, which are perfectly distinct in 

 histological character and in function. 



EcTocYST. — The outer {ectocysi) is essentially a simple 

 chitinous membrane, without any apparent structure, and 

 is a secretion from the inner layer of the cell-wall {endo- 

 cyst). In a large proportion of cases the ectocyst is 

 strengthened by the deposition of calcareous or siliceous 

 particles, and forms a solid wall, the free surface of which 

 is often curiously sculptured or embossed. In others it 

 retains its primitive membranous character, or is slightly 

 coated with earthy material ; in others, again, it is soft 

 and gelatinous. In many forms the calcification of the 

 ectocyst is only partial ; and important systematic distinc- 

 tions are founded on the difPerences in degree which it 

 exhibits, and the consequent structural peculiarities of the 

 cell. 



The Polyzoan colony, as already remarked, consists of 

 a larger or smaller number of zocecia variously united to- 

 gether, which are produced by repeated gemmation. Buds 

 are given off from the primary cells, which are soon deve- 

 loped into secondary cells ; and from these again others 

 arise. In the same way every successive zooecium multi- 

 plies itself ; and the ccEnoecium continues to enlarge so 



* According to Alliuau, the freshwater genus Crisftaella is destitute of an 

 ectocyst. Hyatt, however, regards it as possessing one, and describes it as " a 

 transient gelatinous excretion '' {op. cif. page 14). 



