POLYZOA AXD TUNICATA, 



235 



not yield any nutriment to the capture r, yet by becoming 

 the centre of a crowd of busy Infusoria, multitudes of 

 "which would constantly be drawn into the tentacular vor- 

 tex and swallowed, it would be ancillary to its support, 

 and the organ in question would thus play no unimportant 

 part in the economy of the animal. 



Many members of this Class possess organs analogous 

 to these, but differing considerably in the details of their 

 construction ; and some are also furnished with long stiff 

 bristles, which, moving freely on a joint at the base, can 

 be made, at the will of the animal, to sweep across the 

 face of the cell with considerable force, perhaps with the 

 view to clear it of any extraneous matters that might 

 otherwise annoy or hurt it. Some genera, which have 

 no organs answering to these, inhabit membranous cells 

 instead of shelly ones, as Bowerbarikia and its allies ; in 

 others, the cells- are immersed in a firm fleshy mass, as is 

 the case with the Alcyonidiacice. 



From the Polyzoa the transition is easy and short to 

 the Tunicata. The essential structure is the same in 

 both. The body consists of a sac, with two orifices, usually 

 placed near together, or the discharging one on one side, 

 and a little below the receiving orifice. The circle of ten- 

 tacles around the mouth of the Polyzoan is reduced in the 

 Tunicate (^Ascidia) to short processes, which guard the 

 orifice, and a similar circle is conferred upon the dis- 

 charging one. On the other hand, there now appears a 

 distinct breathiug organ in the form of a hanging bag of 

 membrane, the inner surface of which is covered with 

 oblong cells, set in rows, and these cells are lined with 

 cilia, whose movements resemble those of a toothed wheel. 



