HOW FISHES BREATHE. 
349 
each of which communicates by fissures both with the gullet 
and with the water without. With two exceptions, the names 
of which — Hexanchus and Heptanchus — are sufficiently ex- 
planatory, there are five gill-sacs present, the hinder of which 
contains only a single gill, attached (see Fig. 9) to its front wall. 
The water taken in for respiratory purposes is, in default of 
a branchiostegal and opercular machinery, directed and made 
to move on by muscles and elastic structures which act more 
or less directly upon the gill-chambers and their fissures of 
entrance and exit. 
Is it not possible — rthis is merely thrown out as a suggestion 
— that this peculiar mechanism for breathing, so different in 
action from, though but a modification in structure of, that in 
bony fishes, may, apart from all considerations of embryology 
or of correlation of growth, bear some relation to that peculiar 
habit the fishes in which it occurs have of turning on their 
backs when they take their prey — a proceeding necessitated by 
the position of the mouth on the ventral surface of the body ? 
Sharks, in their foetal state, have temporary external gills, con-' 
sisting of numerous elongated filaments, which project from the 
branchial apertures, immediately in front of the pectoral fins.* 
7 . The respiratory currents pass both in and out of the 
external branchial apertures. 
The breathing apparatus of the Lampreys and certain marine 
fishes whose very name is sufficiently repulsive, the Glutinous 
Hags,f comes under this category. In these fishes the gills 
consist of closed sacs ( b r, b r, Fig. 1 1 ) — not answering, however 
(see explanation of Fig. 11 ), to the gill-chambers of sharks 
and rays — thrown into folds in their interior. These, in the 
Hag, have tubular connections on either side with a duct which 
opens upon the belly of the fish. Betwixt the openings of the 
two ducts lies a third pore, which communicates with the 
gullet, which, in its turn, has a separate tubular communication 
(see Fig. 11) with each gill-sac. The water, then, through the 
middle orifice passes into the gullet, traverses the gill-chambers, 
and finds its way out by the two ducts. The sacs, however, 
have each a small duct which opens by a distinct orifice in the 
skin in the species of Hag called from this circumstance Hep- 
tatrema (see Fig. 11), while in the lampreys, besides possessing 
* See Preparation No. 1061, Koyal College of Surgeons. 
t The Hags, like a certain section of the human community which lives 
upon those of its fellow-mortals who are 11 in difficulties,” bore into the 
bodies of other fish, entering the mouths of their victims when they are 
struggling on the hook of the fisherman. 
The Lampreys and Hags, once called, from the formation of their mouths, 
Cyclostomi , are now included under the term Marsipobranchii (jxapanroQ, a 
pouch, and /3 payx ia t gills)* 
