HOW FISHES BREATHE. 
353 
Royal College of Surgeons. It is a semi-diagrammatic render- 
ing of the branchial arches and their relations to the roof and 
floor of the mouth in the Pike (JEsox Lucius ) as they are seen 
through the widely-opened jaws of the fish. 
In the endeavour to make plain as much as possible which 
. lies in a confined space, some violence has been done to per- 
spective. Nevertheless, the drawing is fundamentally correct. In 
order that the figure may not be more obscure than it is feared 
it may possibly be already, the gill-plates and inwardly-directed 
fringes of teeth have been omitted. 
G Tube of the gullet. 
t Rudimentary tongue. 
p s Parasphenoid bone. 
The rest of the letters have the same signification as those in 
the preceding figure. 
The dotted parts of the diagram indicate those regions of the 
threshold of the gullet which carry a rasp-like armature of 
small teeth. 
Fig. 4. Taken from part of the right side of the specimen — a dried and 
varnished head of a Pike — on which the preceding diagram is 
based. The operculum and branchiostegal rays and membrane 
are here fairly shown. 
o Operculum, 
s o Suboperculum. 
p o Preoperculum. 
io Interoperculum. 
c h Ceratohyal, which joins anteriorly the median, tongue- 
bearing basihyal. 
e h Epihyal, which is joined on behind to the slender stylohyal, 
which, in its turn, articulates with the hyo-mandibular bone : 
all which relations are concealed by the overlapping operculum. 
The cerato- and epi-hyals are each seen to carry seven 
branchiostegal rays (b s'), the former on its inner, the latter on 
its outer side. 
m Lower part of mandible. A flexible membrane, better 
seen in Fig. 1, fills up the gap between it and the upper edge of 
the cerato- and epi-hyals. 
Fig. 5. After Cuvier {Hist. Nat. des Poissons , tome i. pi. viii. fig. 6), repre- 
sents a transverse section of the gill-bearing elements of a 
branchial arch of a Perch, together with its attached gill-plate 
(“feuillet,” Cuv., or u branchial lamina ”), which is seen to be 
made up of two leaflets. 
b r Bone of the arch in section. 
b a Branchial artery in section. It may be observed to divide' 
into two branches, one for each gill-plate, each of which courses 
along the inner edge of a leaflet, giving off in its course nume- 
rous slender twigs. The purified blood is returned to the 
branchial vein — which is seen in section at b v — by two trunks 
which run each along the outer edges of the two leaflets which 
are the components of a gill-plate, having been first collected in 
small vessels which meet the arterial twigs on the mutual terri- 
