18 MOLLUSCA. 
walls of which the extremities of the pulmonary artery are 
spread. This cavity communicates externally by an aper- 
ture which the animal can open or shut at pleasure. 
The mollusca which breathe by means of gills, exhibit 
very remarkable differences, in their number, structure, and 
position. In some cases, there is a single cavity communi- 
cating by an aperture, through which the water enters. 
The walls of this cavity exhibit an uneven surface, dispos- 
ed in ridges, which are the gills, and on which the pulmo- 
nic artery is expanded. This structure exhibits itself in 
the Gasteropoda pectini-branchia. In many cases the gills, 
though seated in a cavity, like the former, and equally ex- 
posed to the contact of the surrounding element, are two in 
number, one on each side, as in the Cephalopoda. In the 
Bivalvia, they are four in number, two on each side like 
leaves, and exterid the whole length of the body. In these, 
the water is admitted at the pleasure of the animal. 
The gills of other mollusca are seated externally, and 
consist either of aborescent productions, or simple cuticular 
elongations, within which the pulmonary artery terminates. 
In some of these, as the Péeropoda, the branchial surface is 
constantly exposed to the action of the surrounding water ; 
whilst in others, the cuticular expansions, which are analo- 
gous to gills, are retractile at the will of the animal. In 
several examples, these cuticular elongations, which execute 
the functions of respiration, are covered with moveable cilia, 
well calculated to excite currents in the water, thus renew- 
ing the portions applied to the surface. 
By means of the characters furnished by the circulating 
and respiratory systems, the molluscous animals may be 
divided into several distinct classes. But as we shall em- 
ploy these characters in the construction of the different 
