434 ZOOLOGY. 
from the level of the pectoral fins to just before the anus, and 
following the upper edge of the body-cavity upward and for- 
ward cut away the body-wall, taking care not to injure the 
large swimming-bladder above, nor the heart in front. Now 
open the pericardial cavity, which lies ventrally immedi- 
ately behind the gills (see Fig. 396, Ht). Cut away the mus- 
cular masses around the back of the head ; expose the cavity 
of the brain, and remove the loose cellular tissue around the 
nervous centres. If the gills of one side are excised and the 
intestine drawn out, the dissection will appear very much as 
in Fig. 396. 
The cavity of the mouth widens rapidly and continues as 
the branchial chamber or pharynx (G'), whence we can pass a 
probe outward through any of the gill-slits. There is a single 
row of sharp-pointed teeth in front on both the under and 
upper jaws; in the pharynx above and below there are 
rounded teeth. At the side of the pharynx are the four gill- 
slits and the four arches ; the inner surface of the anterior 
three arches is smooth, while the arch behind the fourth slit 
is much modified in shape and is armed with tubercles 
and teeth. The entrance of each slit is guarded in front 
and behind by a row of projecting tubercles appended to the 
arches. On the outside of each arch, except the fourth, is 
a double row of filaments, richly supplied with blood-vessels 
which, shining through, give a brilliant red color to the 
gills; on the fourth arch there is but a single row. At the 
upper and posterior corner of the pharynx is the small open- 
ing of the short cesophagus. The branchial chamber has an 
upward extension on the sides of which lie the pseudobran- 
chiz (Ps), accessory respiratory organs not connected with 
the gills proper, and receiving their blood-supply from distinct 
arteries. There are no salivary glands. 
The esophagus dilates almost immediately to form the 
stomach (partly concealed in the figure by the liver, Zz), 
which seems hardly more than a dilatation of the intestine 
(dn). This last isof nearly uniform size throughout, and after 
making three or four coils terminates at the anus, immedi- 
ately in front of the urinary and genital apertures. When 
in situ, the terminal portion of the intestine or-the rectum 
