THE DISSECTION OF THE FISH
=The Blue-green Sunfish.=--The organs found in the abdominal cavity of
the fish may be readily traced in a rapid dissection. Any of the bony
fishes may be chosen, but for our purposes the sunfish will serve as
well as any. The names and location of the principal organs are shown
in the accompanying figure, from Kellogg's Zoology. It represents the
blue-green sunfish, _Apomotis cyanellus_, from the Kansas River, but
in these regards all the species of sunfishes are alike. We may first
glance at the different organs as shown in the sequence of dissection,
leaving a detailed account of each to the subsequent pages.
=The Viscera.=--Opening the body cavity of the fish, as shown in the
plate, we see below the back-bone a membranous sac closed and filled
with air. This is the air-bladder, a rudiment of that structure which
in higher vertebrates is developed as a lung. The alimentary canal
passes through the abdominal cavity extending from the mouth through
the pharynx and ending at the anus or vent. The stomach has the form
of a blind sac, and at its termination are a number of tubular sacs,
the pyloric cæca, which secrete a digestive fluid. Beyond the pylorus
extends the intestine with one or two loops to the anus. Connected with
the intestine anteriorly is the large red mass of the liver, with its
gall-bladder, which serves as a reservoir for bile, the fluid the liver
secretes. Farther back is another red glandular mass, the spleen.
In front of the liver and separated from it by a membrane is the heart.
This is of four parts. The posterior part is a thin-walled reservoir,
the sinus venosus, into which blood enters through the jugular vein
from the head and through the cardinal vein from the kidney. From the
sinus venosus it passes forward into a large thin-walled chamber, the
auricle.
[Illustration: FIG. 16.--Dissection of the Blue-green Sunfish,
_Apomotis cyanellus_ Rafinesque. (After Kellogg.)--27.] Next it flows
into the thick-walled ventricle, whence by the rhythmical contraction
of its walls it is forced into an arterial bulb which lies at the base
of the ventral aorta, which carries it on to the gills. After passing
through the fine gill-filaments, it is returned to the dorsal aorta,
a large blood-vessel which extends along the lower surface of the
back-bone, giving out branches from time to time.
The kidneys in fishes constitute an irregular mass under the back-bone
posteriorly. They discharge their secretions through the ureter to
a small urinary bladder, and thence into the urogenital sinus, a
small opening behind the anus. Into the same sinus are discharged the
reproductive cells in both sexes.
In the female sunfish the ovaries consist of two granular masses of
yellowish tissue lying just below and behind the swim-bladder. In the
spring they fill much of the body cavity and the many little eggs can
be plainly seen. When mature they are discharged through the oviduct to
the urogenital sinus. In some fishes there is no special oviduct and
the eggs pass into the abdominal cavity before exclusion.
In the male the reproductive organs have the same position as the
ovaries in the female. They are, however, much smaller in size and
paler in color, while the minute spermatozoa appear milky rather than
granular on casual examination. A _vas deferens_ leads from each of
these organs into the urogenital sinus.
The lancelets, lampreys, and hagfishes possess no genital ducts. In
the former the germ cells are shed into the atrial cavity, and from
there find their way to the exterior either through the mouth or the
atrial pore; in the latter they are shed directly into the body cavity,
from which they escape through the abdominal pores. In the sharks and
skates the Wolffian duct in the male, in addition to its function as
an excretory duct, serves also as a passage for the sperm, the testes
having a direct connection with the kidneys. In these forms there is a
pair of Müllerian ducts which serve as oviducts in the females; they
extend the length of the body cavity, and at their anterior end have an
opening which receives the eggs which have escaped from the ovary into
the body cavity. In some bony fishes as the eels and female salmon the
germ cells are shed into the body cavity and escape through genital
pores, which, however, may not be homologous with abdominal pores. In
most other bony fishes the testes and ovaries are continued directly
into ducts which open to the outside.
=Organs of Nutrition.=--The organs thus shown in dissection we may now
examine in detail.
[Illustration: FIG. 17.--Black Swallower, _Chiasmodon niger_ Johnson,
containing a fish larger than itself. Le Have Bank.]
The mouth of the fish is the organ or series of structures first
concerned in nutrition. The teeth are outgrowths from the skin,
primarily as modified papillæ, aiding the mouth in its various
functions of seizing, holding, cutting, or crushing the various kinds
of food material. Some fishes feed exclusively on plants, some on
plants and animals alike, some exclusively on animals, some on the mud
in which minute plants and animals occur. The majority of fishes feed
on other fishes, and without much regard to species or condition. With
the carnivorous fishes, to feed represents the chief activity of the
organism. In proportion to the voracity of the fish is usually the size
of the mouth, the sharpness of the teeth, and the length of the lower
jaw.
The most usual type of teeth among fishes is that of villiform
bands. Villiform teeth are short, slender, even, close-set, making
a rough velvety surface. When the teeth are larger and more widely
separated, they are called cardiform, like the teeth of a wool-card.
Granular teeth are small, blunt, and sand-like. Canine teeth are those
projecting above the level of the others, usually sharp, curved, and
in some species barbed. Sometimes the canines are in front. In some
families the last tooth in either jaw may be a "posterior canine,"
serving to hold small animals in place while the anterior teeth crush
them. Canine teeth are often depressible, having a hinge at base.
[Illustration: FIG. 18.--Jaws of a Parrot-fish, _Sparisoma
aurofrenatum_ (Val.). Cuba.]
Teeth very slender and brush-like are called setiform. Teeth with
blunt tips are molar. These are usually enlarged and fitted for
crushing shells. Flat teeth set in mosaic, as in many rays and in the
pharyngeals of parrot-fishes, are said to be _paved_ or tessellated.
Knife-like teeth, occasionally with serrated edges, are found in
many sharks. Many fishes have incisor-like teeth, some flattened
and truncate like human teeth, as in the sheepshead, sometimes with
serrated edges. Often these teeth are movable, implanted only in
the skin of the lips. In other cases they are set fast in the jaw.
Most species with movable teeth or teeth with serrated edges are
herbivorous, while strong incisors may indicate the choice of snails
and crabs as food. Two or more of these different types may be found
in the same fish. The knife-like teeth of the sharks are progressively
shed, new ones being constantly formed on the inner margins of the jaw,
so that the teeth are marching to be lost over the edge of the jaw as
soon as each has fulfilled its function. In general the more distinctly
a species is a fish-eater, the sharper are the teeth. Usually fishes
show little discrimination in their choice of food; often they devour
the young of their own species as readily as any other. The digestive
process is rapid, and most fishes rapidly increase in size in the
process of development. When food ceases to be abundant the fishes grow
more slowly. For this reason the same species will grow to a larger
size in large streams than in small ones, in lakes than in brooks. In
most cases there is no absolute limit to growth, the species growing as
long as it lives. But while some species endure many years, others are
certainly very shortlived, and some may be even annual, dying after
spawning, perhaps at the end of the first season.
Teeth are wholly absent in several groups of fishes. They are, however,
usually present on the premaxillary, dentary, and pharyngeal bones.
In the higher forms, the vomer, palatines, and gill-rakers are rarely
without teeth, and in many cases the pterygoids, sphenoids, and the
bones of the tongue are similarly armed.
No salivary glands or palatine velum are developed in fishes. The
tongue is always bony or gristly and immovable. Sometimes taste-buds
are developed on it, and sometimes these are found on the barbels
outside the mouth.
[Illustration: FIG. 19.--Sheepshead (with incisor teeth), _Archosargus
probatocephalus_ (Walbaum). Beaufort, N. C.]
=The Alimentary Canal.=--The mouth-cavity opens through the pharynx
between the upper and lower pharyngeal bones into the oesophagus,
whence the food passes into the stomach. The intestinal tract is in
general divided into four portions--oesophagus, stomach, small and
large intestines. But these divisions of the intestines are not always
recognizable, and in the very lowest forms, as in the lancelet, the
stomach is a simple straight tube without subdivision.
In the lampreys there is a distinction only of the oesophagus with many
longitudinal folds and the intestine with but one. In the bony fishes
the stomach is an enlarged area, either siphon-shaped, with an opening
at either end, or else forming a blind sac with the openings for
entrance (cardiac) and exit (pyloric) close together at the anterior
end. In the various kinds of mullets (_Mugil_) and in the hickory shad
(_Dorosoma_), fishes which feed on minute vegetation mixed with mud,
the stomach becomes enlarged to a muscular gizzard, like that of a
fowl. Attached near the pylorus and pouring their secretions into the
duodenum or small intestine are the _pyloric cæca_. These are tubular
sacs secreting a pale fluid and often almost as long as the stomach or
as wide as the intestine. These may be very numerous as in the salmon,
in which case they are likely to become coalescent at base, or they be
few or altogether wanting.
Besides these appendages which are wanting in the higher vertebrates, a
pancreas is also found in the sharks and many other fishes. This is a
glandular mass behind the stomach, its duct leading into the duodenum
and often coalescent with the bile duct from the liver. The liver in
the lancelet is a long diverticulum of the intestine. In the true
fishes it becomes a large gland of irregular form, and usually but not
always provided with a gall-bladder as in the higher vertebrates. Its
secretions usually pass through a _ductus cholodechus_ to the duodenum.
The _spleen_, a dark-red lymphatic gland, is found attached to the
stomach in all fish-like vertebrates except the lancelet.
The lining membrane of the abdominal cavity is known as the
_peritoneum_, and the membrane sustaining the intestines from the
dorsal side, as in the higher vertebrates, is called the _mesentery_.
In many species the peritoneum is jet black, while in related forms it
may be pale in color. It is more likely to be black in fishes from deep
water and in fishes which feed on plants.
=The Spiral Valve.=--In the sharks or skates the rectum or large
intestine is peculiarly modified, being provided with a spiral valve,
with sometimes as many as forty gyrations. A spiral valve is also
present in the more ancient types of the true fishes as dipnoans,
crossopterygians, and ganoids. This valve greatly increases the
surface of the intestine, doing away with the necessity for length.
In the bowfin (_Amia_) and the garpike (_Lepisosteus_) the valve is
reduced to a rudiment of three or four convolutions near the end of the
intestine. In the sharks and skates the intestine opens into a cloaca,
which contains also the urogenital openings. In all fishes the latter
lie behind the orifice of the intestine. In the bony fishes and the
ganoids there is no cloaca.
[Illustration: FIG. 20.--Stone-roller, _Campostoma anomalum_
(Rafinesque). Family _Cyprinidæ_. Showing nuptial tubercles and
intestines coiled about the air-bladder.]
=Length of the Intestine.=--In all fishes, as in the higher
vertebrates, the length of the alimentary canal is coordinated with the
food of the fish. In those which feed upon plants the intestine is very
long and much convoluted, while in those which feed on other fishes it
is always relatively short. In the stone-roller, a fresh-water minnow
(_Campostoma_) found in the Mississippi Valley, the excessively long
intestines filled with vegetable matter are wound spool-fashion about
the large air-bladder. In all other fishes the air-bladder lies on the
dorsal side of the intestinal canal.