258 



POLYZOA INFUNDIBULATA. 



vidual is a distinct organism, and the medium which binds 

 them together, whether vascular or ligamentous, has its own 

 peculiar character. The one we may compare to a chain 

 of which all the links are welded, the other to a necklace 

 where the beads are strung together by a percurrent thread. 



The body of the Polyzoa is lengthened, somewhat cylin- 

 drical, or at times bulged at the base, and when at rest lies, 

 Fig. 59. in the form of a syphon, doubled up upon 



itself in the cell, (Fig. 59 ,) to which it is 

 connected by a tendon at the bottom, and 

 by the duplicature of a thin membrane 

 round the aperture, so that it is impos- 

 sible it should ever voluntarily leave the 

 cell to swim at large, as Baster and others 

 have maintained. The head or upper end 

 is surrounded by a single row of tenta- 

 cula, (Fig. 60, #), which are tubular, fili- 

 form and non-contractile, for the animal 

 can only shorten them, excepting to a 

 slight extent, by rolling them up in a spiral manner. 

 They are apparently smooth, but with a high magnifier 

 it is ascertained that they are clothed 

 with numerous fine cilia,* which are 

 in ceaseless motion, and are supposed 

 to perform the office of breathing or- 

 gans by keeping up a constant cur- 

 rent of water along their surfaces. The 

 current sets in towards the mouth in 

 an invariable direction ; and from the 

 incessant revolution of particles with- 

 in the mouth and the gullet, observed 

 by Professor Grant, this organ seems 

 to be also ciliated internally. The 

 more especial use of the tentacula is 

 to arrest the prey which chance floats 

 within their reach and conduct it to the mouth, a simple 

 aperture placed in the centre of the tentacular circle, and 



* For a history of this discovery, written with great learning and impartiality, see 

 Dr. Sharpey's article " Cilia," in the Cyclopaedia of Anatomy and Physiology, vol. i. 

 p. 609. 



