4:64 THE MICROSCOPE. 



fiion of a " cell," surroiuKling the base of each polype, and 

 usually capable of recciviug it when retracted. The do- 

 velopmcDt of this cell at ouce distinguishes it from the 

 not altogether dissimilar group, the Diphydce and Physo- 

 Ifhorkhe. In some Sertulariadce (Sertularia dynamcna), 

 the margins of the cells are converted into membranous 

 valves; and in the genus Plumidariaj we find special 

 orjians of offence. 



The Diphydce are among the most remarkable and 

 beautiful inhabitants of the ocean, to whose warmei* 

 regions, they, like the Fhysoj^horidcr, 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 natatorir.l 

 organs with v>dnch they are provided, they possess no 

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

 veloped in the Physophoridce. The tentacle of the Diphydie 

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

 elono-ation 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 ofi'ence. 

 Three genera of the Physoplioridce are particularly w^orthy 

 of notice : the Physalia, whose air-vesicle mcay 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 

 Vclella 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 Veleila 

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

 Physop/iorld. In the very peculiar genus Lucernaria, — 

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

 liave a Ilydrozoon, in which the polype occupies the ceuiro 

 of an expanded disc, the two presenting essentially the same 

 structure and relations as in the Medusiform zooids of 

 other divisions. In fiict, 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 Lucerne riadw to that of 



