xlii INTRODUCTION. 



gelatinous covering*, and borne at the summit of the 

 capsule, in which they complete their development. 

 (Woodcut, fig. xvii.) This species of nidification is far from 

 uncommon. The marsupium is sometimes formed, as 

 Allman has suggested, by an extension of the endotheca 

 or membrane which immediately confines the ova (Wood- 

 cut, fig. xvii.) ; but in other cases the entire gonozooid is 

 pushed upward, and at last beyond the opening of the 

 capsule, by the growth of the column supporting it, and, 

 having secreted a gelatinous coating, is converted into a 

 kind of nest, in which the ova pass through the later stages 

 of their development f. 



It may be well briefly to sum up the leading facts of the 

 reproductive history of the Hydroida. 



In each colony, the alimentary and reproductive func- 

 tions are distributed amongst two classes of zooids. 



The sexual zooids, like the flower-bud of the plant, are 

 only developed at certain seasons, and occupy various 

 positions in the different species. In a large number 

 of cases they exhibit a modification of structure adapt- 

 ing them for independent existence, and when mature 

 detach themselves from the colony and become free and 

 locomotive. 



The free sexual zooid, in all but one or two exceptional 

 cases, may be regarded as essentially a polypite with its 

 arms united by a contractile web, so as to form a float and 

 natatory organ. Disguised by its adaptive dress, it has 

 been separated from its kindred under the name of a 

 medusa; it is in reality a swimming polypite. 



* The acrocyst of Allman. 



I' Wright has observed this in Opercularella /nc/ruttr. The marsupium of 

 Campamdaria neglectu is formed in the same way. ( Vule p. 172.) 



