HOW FISHES BKEATHE. 
343 
behind its tip, and to be met simultaneously by a similar 
screen which rises from behind the tip of the lower jaw. This, 
as Cuvier suggests, has probably the effect of preventing any 
reflux of the water which has just been taken in, and possi- 
bly, which Professor Owen doubts, the escape also of food from 
the mouth. 
Simultaneously with the appearance of this curtain, or 
66 velum,” a peculiar kind of spiny ruff ( b s, Figs. 1 and 4), pre- 
sently to be described, seen around the throat, so to speak, of 
the fish, expands, and almost immediately the mouth is shut. 
The above action, seemingly so simple both to witness and 
to describe, presents one difficulty and complexity to the ob- 
server, viz. that he has to keep his eye on two points which 
are somewhat wide apart, namely, the u velum ” in the mouth 
and the prickly ruff around the throat of the fish. Hence may 
result an error differing only from the “ personal equation ” of 
the astronomer, in that it concerns but one sense, for which 
some allowance must be made. 
As the observer, unless he has no respect unto his fingers, 
will not care to make very prolonged explorations into the 
interior of the mouth of a living fish, at all events of the kind 
which has been instanced, he must content himself with 
making further observations on a dead specimen. 
In considering the various modifications of the breathing 
organs of fishes, it will be found practically convenient to 
follow the arrangement adopted by Prof. Owen in his Hunterian 
Catalogue of the contents of the Museum of the Royal College 
of Surgeons. 
a. The respiratory currents enter by the mouth and are 
expelled by an orifice on each side. % If we take a pike and 
look down its widely opened mouth, we shall see at the back of 
the throat something like that which has been represented, 
diagram fashion, in Fig. 3. At the front of the floor of the 
mouth will be seen a small tongue, not flexible and protrusive 
as in ourselves, but having its range of motion limited by that 
of the median chain of bones of which it forms the tip. On 
either side of the narrow floor, which is carpeted here and 
there (see dotted part of the figure) by rasp-like sets of very 
dont l’effet doit etre d’empecher les alimens et surtout l’eau avalee pour la 
respiration, de ressortir par la bouche .” — Histoire Nciturelle des Poissons, 
vol. i. p. 497. 
Tbis organ, from a kind of remote analogy to the velum pendulum palati , 
a structure which, with the tonsils, is absent in all fishes, may conveniently 
be termed a velum.” 
* Pishes in which this occurs were once termed, for reasons which we 
shall see presently, “pisces branchiis liberis.” Most of them have a bony 
skeleton. 
A A 2 
