8.5 



fine! tliat the external characters of the Radiala are enn;raftiMl 

 on a type of tnnicated Mollusca : characters, thoui^h distinct 

 from each, wliich yet soften away into both. Some of the 

 polypes of this order have been arranged by most auihors 

 among the IJydroidns, in consequence of llieir polypidoms 

 resembling those of the Sertulariad^e, By the examination 

 of their polypes, however, they are novv transfered to the 

 MoUuscan zoophytes; and this grouping seems far from 

 being unnatural, even when their external forms are con- 

 sidered. Whatever alteration may be made in the future 

 arrangement of these creatures, when they shall have been 

 more thoroughly studied, yet the making the polype the 

 foundation on which to rest, is the only true one to guide 

 the systematist. The order is characterized as " Pnli/pes 

 aggregate, the mouth encircled ivith Jilifurm, ciliated, re- 

 tractile tentacula j stomach distinct, tvith a curved intestine 

 terminating in an anus near the mouth ; ova internal. Poly' 

 pidoms very variable j either horny, Jistular, and confervoid, or 

 membranous or Jibro-gelatinous ; formed of cells connettcd and. 

 arranged in a determinate, and usually quincunciul manner." 

 As the polype, therefore, forms the foundation of the ar- 

 rangement, it will be necessary to describe it, though without 

 entering into minute detail. In its expanded state it stands 

 prominently from the cell; it is columnar and transparent, 

 so that the internal structure can be seen. Between the sur- 

 face of the polype and the internal organs, there is an 

 intervening space, similar to what has been noticed in the 

 Asteroitl zoophytes. This space is said to be occupied by 

 a clear fluid; and here also are the muscles, by which the 

 polype effects its various and rapid motions. The upper 

 portion of the column is surrounded with numerous long 

 slender tentacula. In the foregoing orders it has been found 

 that the chief uses of the tentacula were for the capture of 

 prey ; here their functions have undergone a complete revo- 

 lution, for to tliem belongs the function of respiration, and 

 in a secondary degree only that of nutrition. If an expan- 

 ded polype be examined with a microscope, it will be found 

 to resemble a minute and delicate flower endowed with sen- 

 sation and voluntary motion. The tentacula which are long 

 and slender, are cloalhed with nunierous minute cilia, which 

 are in a constant state of activity. By their vibrations 

 numerous currents pass over the tentacula and across the 

 oral surface. Any insect or other substance coming within 

 these currents, passes over the mouth, and in so doing it 

 is instantly caught by the prehensile lips, which are ex- 

 ceedingly sensitive, and conveyed to the stomach. Thus 

 we see both offices are performed by one set of organs, one 

 being made subservient to the other. These tentacula, like 



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