POLYZOA AND TUNIOATA. 
235 
not yield any nutriment to the capturer, yet by becoming 
the centre of a crowd of busy Infusoria, multitudes of 
which would constantly be drawn into tho 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 tho 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 Bowerbanlcia and its allies ; in 
others, the cells are immersed in a firm fleshy mass, as is 
the case with tho Alcyonidiadm. 
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 roducod 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 breathing organ in tho form of a hanging bag of 
membrane, the inner surface of whioh is covered with 
oblong cells, set in rows, and these cells are lined with 
cilia, whose movements resemble those of a toothed wheel. 
