POLYPIFERA. 345 



sion of a " cell," surrounding the base of each polype, and 

 usually capable of receiving it when retracted. The de- 

 velopment of this cell at once distinguishes it from the 

 not altogether dissimilar group, the Diphydce and Physo- 

 phoridce. In some Sertulariadce (Sertularia dynamena), 

 the margins of the cells are converted into membranous 

 valves; and in the genus Plumularia, we find special 

 organs of offence. 



The Diphydce are among the most remarkable and 

 beautiful inhabitants of the ocean, to whose warmer 

 regions, they, like the Physophoridce, are principally con- 

 fined. They are free swimmers in the adult state, and 

 probably, at all times of their existence ; but while actively 

 locomotive by means of the contractions of the natatorial 

 organs with which they are provided, they possess no 

 special supporting apparatus, or " float," such as that de- 

 veloped in the Physophoridce. The tentacle of the Diphydce 

 is a long filiform process of the peduncle, capable of great 

 elongation and contraction ; the terminal filament of 

 which is closely beset with minute thread-cells, so that the 

 whole must constitute a very efficient weapon of offence. 

 Three genera of the Physophoridce are particularly worthy 

 of notice : the Physalia, whose air-vesicle may attain the 

 length of eight or nine inches, while its formidable ten- 

 tacles hang down for as many feet, inflicting instantaneous 

 death upon the smaller animals, and giving rise to no small 

 amount of pain and irritation, even in man ; and the 

 Velella and Porpita, in which the texture of the air-vesicle 

 is so exceedingly firm, as to give it the appearance of an 

 internal shell, while its cavity is subdivided into numerous 

 chambers. Originally, however, the " shell " of the Velella 

 is a perfectly simple air-vesicle, like that of any other 

 Physophorid. In the very peculiar genus Lucernaria, 

 lovely " Lamp-polype," with little knobbed tentacles we 

 have a Hydrozoon, in which the polype occupies the centre 

 of an expanded disc, the two presenting essentially the same 

 structure and relations as in the Medusiform zooids of 

 other divisions. In fact, a Lucernaria is in all essential 

 respects comparable to an Aurelia, or other Medusa fixed 

 by the middle of the upper surface of its disc. 



Mr. Huxley prefers the term Lucernariadce to that of 



