HTDROZOA. 43 



tween the central and peripheral portions of the 

 thickened boss, commencing at the distal ex- 

 tremity, and extending down very nearly to the 

 proximal end of the boss, so as to leave the thick 

 central portion, enclosing the central cavity, at- 

 tached to the thin peripheral portion (which re- 

 mains as the wall of the cavity of the bell) only at 

 its proximal extremity. In the perfect condition 

 of the zooid thus produced, the endoderm lines 

 the cavity of the peduncle by which it is attached, 

 the canals, and the central cavity of the suspended 

 axial body ; while the ectoderm forms the whole 

 of the outer walls of both natatorial organ and 

 central sac." 



Still further changes are liable to ensue. The 

 central sac, or * manubrium,' may acquire a mouth 

 at its distal extremity, thus, as it were, transform- 

 ing itself into a polypite, while the natatorial organ, 

 or ^ gonocalyx,' enlarging, loosens its attachment, 

 and swims freely in the sea as a veritable Medusid 

 ( fig. 6, in). Indeed, there is every reason to be- 

 lieve that a great majority of the organisms de- 

 scribed as Medusidce are, in reality, the detached 

 reproductive bodies of other Hydrozoa {figs. 13 

 and 14). 



Such bodies, however, are more than mere 

 organs. Many of them, when first liberated, pre- 

 sent no distinct traces of generative elements, 

 pending the formation of which essential products, 

 their independent existence is of necessity pro- 

 longed. At this period they lead a very active 

 life, incre se rapidly in size, and eagerly devour 

 such minute marine animals as they are able to 

 secure. During the calmer seasons of the year 

 they abound in our seas, but before the approach 



