PISCES. 
In the Sharks the orifices of the pharynx 
leading into the branchial chambers are guarded 
by cartilaginous pyramidal processes ; but in 
the Skates, which have the cartilaginous arches 
much less perfectly formed, no such defences 
are visible. 
The Cyclostomatous cartilaginous Fishes, 
from the peculiarities of their habits, require 
another modification in the construction of 
the organs of respiration, seeing that, whilst 
they rest fixed by their suctorial mouths to 
the surfaces of stones or other foreign bodies, 
or while they are compelled to remain with 
their heads deeply plunged into the flesh of 
the prey upon which they live, the admission 
of water into the mouth and its subsequent ex- 
pulsion through pharyngeal fissures would, in 
their case, be impracticable. 
In the Myxine or Hag-fish the branchial 
apparatus presents externally seven round holes 
placed in a line along each side of the neck, 
and situated very far back in comparison with 
the usual situation of the gill-openings.. Each 
of the seven lateral openings leads internally 
into a flattened circular cavity, which likewise 
communicates by.a special canal with the inte- 
rior of the er. When opened, each gill- 
sac is found to have its lining vascular mem- 
brane gathered into closely set folds, which in 
turn exhibit, when accurately examined, smaller 
plice, and thus a great superficial extent of 
respiratory membrane is secured, much in the 
same manner as in the case of the Raide ; in- 
deed, the general disposition of the whole ap- 
paratus is essentially similar in the two groups, 
the great difference being that whereas in the 
plagiostome genera the water passes in at the 
mouth and out at the gills, in the Cyclosto- 
mata it alternately passes in and out through 
the same orifices by a mechanism which is best 
exhibited in the Lampreys ( Petromyzon ). 
In the last mentioned genus of Cyclosto- 
matous Fishes, the seven holes which lead to 
the breathing organs are seen, on removing the 
skin, to be supported by a curiously contorted 
framework of cartilaginous pieces, which we 
have already noticed as being the exaggerated 
representatives of the sternal ribs met with in 
the Sharks, and which we shall subsequently 
see in a much more highly developed condi- 
tive to the extent of the respiratory surface pre- 
sented by the gills of Fishes to the action of the 
surrounding medium. On each side of the body 
of a Skate, says he, there are four double gills, or 
gills with two sides each, and one single gill; or 
there are in all eighteen sides or surfaces on which 
the branchial artery is spread out. On each of 
these sides there are about fifty divisions or dou- 
blings of the membrane of the gills. Each division 
has on each side of it 160 subdivisions, doublings, or 
folds of its membrane, the length of each of which 
_in a very large skate is about one-eighth of an inch, 
and its breadth about one-sixteenth of an inch, 
so that in the whole gills there are 144,000 subdi- 
visions or folds, the two sides of each of which are 
equal to the 64th part of a square inch, or the sur- 
face of the whole gills in a large skate is equal to 
2.250 square inches, that is, to 15 square feet, an 
expanse equal to the whole external surface of the 
pare body.—Structure and Physiology of Fishes, 
p- 15. 
987 
tion in Branchiostoma. These form a kind of 
elastic thorax around the region of the body 
where the branchie are situated, which is made 
to perform alternate movements of contraction 
and dilatation, whereby the water is perpetually 
sucked in and again expelled through the exter- 
nal openings (fig.521, 2,7). The branchiz them- 
selves (h, h) present nearly the same structure as 
in the Myxine, consisting of as many distinct 
Respiratory apparatus of the Lamprey ( Petromyzon ). 
a, mouth with its teeth; b, phery 5 ¢, opening 
of tube from the back of the head; d, cavity com- 
municating with the respiratory sacculi of both 
sides ; e, commencement of esophagus; f, carti- 
laginous process connected with hyoid tooth, like- 
wise marked d; g, muscles of sucking disc; h, h, 
respiratory cavities and branchiz contained therein ; 
a, i, external openings of do. 3; m, cartilaginous sac 
investing the pericardium. 
sacculi as there are external openings, ranged 
along the sides of the neck, each sacculus having 
its vascular lining membrane finely plicated in 
order to increase the extent of surface to be 
exposed to the respired element. Every bran- 
chial sacculus, in addition to its communica- 
tion with the exterior through the lateral open- 
ings of the neck, has a passage that leads into 
the pharynx (d), so that by the intervention of 
the pharyngeal cavity the respiratory sacculi of 
the opposite sides of the body are made to 
communicate with each other, an arrangement 
which explains the circumstance, that when 
the breathing holes of one side of the Lamprey 
are kept above water, the respired fluid which 
enters the submerged orifices, after traversing 
the pharynx, fills the branchial chambers of the 
gpposite side, and is forcibly ejected therefrom 
