ANATOMY OF FISH. 
155 
an incision along the median ventral line from the level of the pec- 
toral fins to just before the anus, and following the upper edge of the 
body-cavity upward and forward cut away the body-wall, taking 
care not to injure the large swimming-bladder above, nor the heart 
in front. Now open tlie pericardial cavity, which lies immediately 
behind and below the gills (see Fig. 198, Ht). Cut away the muscular 
masses around the back of the head; expose the cavity of the brain, 
and remove the loose cellular tissue around the brain. If the gills 
of one side are cut away and the intestine drawn out, the dissection 
will appear very much as in Fig. 198. 
The cavity of the mouth widens rapidly, becoming behind the bran- 
chial chamber or pharynx, 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 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-ves- 
sels 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 opening of the short oesophagus. 
The branchial chamber has an upward extension on the sides of 
"which lie the false gills (Ps), which are accessory respiratory organs 
