HOW FISHES BREATHE. 
355 
In Heptcinchus there are nine or ten visceral arches. 
Fig. 11. Taken from a preparation (No. 1018) in the Museum of the Royal 
College of Surgeons, represents two of the six gill-sacs on one 
side of the Myxine or “ Glutinous Hag” (Heptatrema cirratum). 
These sacs,* br br, the upper of which is represented in profile, 
are seen internally to communicate with the oesophagus (os), each 
by a short tube. Externally, they are brought into relation, by 
similar means, with one of the two longitudinal canals (not shown 
in the figure) which discharge the de-oxygenated water by two 
pores opening on the ventral surface of the body. The lowest 
arrows show the direction of the water used for respiration, 
which is admitted not at the mouth, as in other fishes, but at a 
small opening placed betwixt the two pores which allow of its 
exit. Each sac is further seen to be supplied with a branch 
from the branchial artery (a) of its side. 
Fig. 12J.S a view, from above, of the respiratory apparatus of the African 
Mud-fish, Lepidosiren (Protopterus) Annectens , reduced from a 
figure in Professor Owen’s paper on the anatomy of this fish. 
(See Trans. Linn. Soc. vol. xviii. tab. 26, fig. 1). 
The pharynx has been slit up in the middle line, and its sides 
spread out on either side. 
The five branchial fissures are indicated by numbers. 
t Tongue. 
g Aperture, or glottis, pierced in the rudimentary thyroid carti- 
lage (t h) leading to the short “ ductus pneumaticus,*’ or trachea 
(dp), which, in turn, communicates with two lung-like sacs. 
v is a valve which, though it is not stated to do so by Professor 
Owen, may act, fo judge from the figure, as an epiglottis, i.e. 
prevent food, &c., from falling into the glottis. 
(All figures founded by the author on specimens in the Museum of the Royal 
College of Surgeons were drawn by the kind permission of Professor 
Flower , F.R.S.~] 
* 11 The leading condition,” says Professor Owen, u of the gills in other 
fishes may be understood by supposing each compressed sac of a Myxine to 
be split through its plane, and each half to be glued by its outer smooth side 
to an intermediate septum, which would then support the opposite halves of 
two distinct sacs, and expose their vascular mucous surface to view. If the 
septum be attached by its entire margin, the condition of the plagiostomous 
gill is effected. If the septum be liberated at the outer part of its circum- 
ference, and the vascular surfaces are produced into pectinated lamelligerous 
processes, tufts, or filaments, proceeding from the free arch, the gill of an 
ordinary osseous or teleostomous fish is formed. Such a gill is the homo- 
logue, not of a single gill-sac, but of the contiguous halves of two distinct 
gill-sacs, in the Myxines. Already, in the Lampreys, the first stage of this 
bi-partition may be seen, and the next stage in the Sharks and Rays.” 
This throws much light upon the somewhat puzzling variations of the 
branchial arteries in fishes. See also Gegenbaur, op. cit. p. 809 ; and the 
Introduction of Professor Rolleston’s Forms of Animal Life. 
