194 Proceedings of the 



ing alimentary and sexual organs polypoid in form. Each 

 polypary is unisexual or dioecious ; male and female organs 

 being never situated on the same polypary, or on the same shell! 

 The polypary, in addition to its alimentary and sexual polyps, 

 possesses additional organs of prehension and offence, hitherto 

 undescribed, which have a determinate situation. It is also 

 furnished with corallum or skeleton, which, although widely 

 differing in form from the coralla of other hydroid zoophytes, 

 is identical with them in chemical composition and mode of 

 secretion. 



Anatomy of Hydractinia. 



4. A consideration of the anatomy of Hydractinia may be 

 conveniently divided into three parts : 



1st, The Corallum or polypidom, the horny skeleton of the 

 zoophyte. 

 2d, The Polypary, or common body. 

 3d, The Polyps. 



The Corallum. 



5. The Corallum is a layer of transparent amber-coloured 

 chitine ; a substance having the consistence of horn, but 

 differing from it in chemical composition. It closely invests 

 that part of the shell inhabited by the zoophyte. Its struc- 

 ture is very dense at the mouth of the shell, where it fre- 

 quently (as remarked by Johnston) forms a slight extension 

 of the whorl. As it passes backward, it gradually takes the 

 form of a fine indurated membrane beset with spicules of 

 irregular shape. Delicate and transparent casts of entire 

 shells are thus occasionally formed, which may be removed 

 uninjured by the aid of consecutive solutions of nitric acid 

 and caustic potash. I have placed one of these on the table ; 

 and it will be seen that the surface, with all its parasitical 

 growths, is copied with the fidelity of the electrotype. Over 

 the more exposed parts of the corallum, the chitine rises up 

 in the form of thickly set papillae or spines, which have their 

 surfaces furrowed by deep longitudinal grooves (fig. 2). The 

 ridges between these grooves are coarsely lobed or serrated, 

 and after passing from the summit to the bases of the spines, 

 traverse the corallum, and unite with each other, and with 

 the ridges of the neighbouring papillae, so as to form a raised 



