THE ORGANS OF NUTRITION AND DIGESTION.
Fishes are either exclusively carnivorous or herbivorous, but not a few
feed on vegetable substances as well as animal, or on mud containing
alimentary substance in a living or decomposing state. Generally they
are very voracious, especially the carnivorous kinds, and the rule of
“eat or be eaten” applies to them with unusual force. They are almost
constantly engaged in the pursuit and capture of their prey, the
degree of their power in these respects depending on the dimensions
of the mouth and gullet and the strength of the teeth and jaws. If
the teeth are sharp and hooked, they are capable of securing the most
slender and agile animals; if this kind of teeth is combined with a
wide gullet and distensible stomach, they are able to overpower and
swallow other fish larger than themselves; if the teeth are broad,
strong molars, they are able to crush the hardest aliments; if they
are feeble, they are only serviceable in procuring some small or inert
and unresisting prey. Teeth may be wanting altogether. Whatever the
prey, in the majority of cases it is swallowed whole; but some of the
most voracious fishes, like some Sharks and _Characinidæ_, are
provided with cutting teeth, which enable them to tear their prey to
pieces if too large to be swallowed whole. Auxiliary organs for the
purpose of overpowering their prey, which afterwards is seized or torn
by the teeth, like the claws of some carnivorous mammals and birds, are
not found in this class; but in a few fishes the jaws themselves are
modified for that purpose. In the Sword-fishes the bones of the upper
jaw form a long dagger-shaped weapon, with which they not only attack
large animals, but also frequently kill fishes on which they feed.
The Saw-fishes are armed with a similar but still more complicated
weapon, the saw, which is armed on each side with large teeth implanted
in deep sockets, specially adapted for killing and tearing the prey
before it is seized and masticated by the small teeth within the mouth.
Fishes show but little choice in the selection of their food, and some
devour their own offspring indiscriminately with other fishes. Their
digestive powers are strong and rapid, but subject in some degree to
the temperature, which, when sinking below a certain point, lowers the
vital powers of these cold-blooded animals. On the whole, marine fishes
are more voracious than those inhabiting fresh waters; and whilst the
latter may survive total abstinence from food for weeks or months,
the marine species succumb to hunger within a few days. The growth of
fishes depends greatly on the nature and supply of food, and different
individuals of the same species may exhibit a great disparity in their
respective dimensions. They grow less rapidly and to smaller dimensions
in small ponds or shallow streams than in large lakes and deep rivers.
The young of coast fishes, when driven out to sea, where they find
a much smaller supply of food, remain in an undeveloped condition,
assuming an hydropic appearance. The growth itself seems to continue
in most fishes for a great length of time, and we can scarcely set
bounds to--certainly we know not with precision--the utmost range of
the specific size of fishes. Even among species in no way remarkable
for their dimensions we sometimes meet with old individuals, favourably
situated, which more or less exceed the ordinary weight and measurement
of their kind. However, there are certain evidently short-lived species
of fishes which attain a remarkably uniform size within a very short
time; for instance, the Stickleback, many species of _Gobius_ and
_Clupea_.
* * * * *
The organs of nutrition, manducation, and deglutition, are lodged in
two large cavities--an anterior (the mouth or _buccal cavity_), and
a posterior (the _abdominal cavity_). In the former the alimentary
organs are associated with those fulfilling the respiratory functions,
the transmission of food to the stomach and of water to the gills
being performed by similar acts of deglutition. The abdominal cavity
commences immediately behind the head, so, however, that an extremely
short thoracic cavity for the heart is partitioned off in front. Beside
the alimentary organs it contains also those of the urogenital system
and the air-bladder. The abdominal cavity is generally situated in the
trunk only, but in numerous fishes it extends into the tail, being
continued for some distance along each side of the hæmal apophyses.
In numerous fishes the abdominal cavity opens outwards by one or two
openings. A single _porus abdominalis_ in front of the vent is found
in _Lepidosiren_ and some Sturgeons; a paired one, one on each side
of the vent, in _Ceratodus_, some species of Sturgeon, _Lepidosteus_,
_Polypterus_, _Amia_, and all Chondropterygians. As in these fishes
semen and ova are discharged by proper ducts, the abdominal openings
may serve for the expulsion of semen, and those ova only which, having
lost their way to the abdominal aperture of the oviduct, would be
retained in the abdominal cavity. In those _Teleosteans_ which lack an
oviduct a single _porus genitalis_ opens _behind_ the vent.
The _mouth_ of fishes shows extreme variation with regard to form,
extent, and position. Generally opening in front, it may be turned
upwards, or may lie at the lower side of the snout, as in most
Chondropterygians, Sturgeons, and some Teleosteans. Vogt regards this
position as a persistent fœtal condition. In most fishes the jaws are
covered by the skin, which, before passing over the jaws, is often
folded, forming more or less fleshy lips. In the Sharks the skin
retains its external character even within the teeth, but in other
fishes it changes into a mucous membrane. A _tongue_ may exist as a
more or less free and short projection, formed by the glosso*-hyal and
a soft covering, or may be entirely absent. _Salivary_ glands and a
_velum palati_ are absent in fishes.
With regard to the _dentition_, the class of Fishes offers an amount
of variation such as is not found in any of the other classes of
Vertebrates. As the teeth form one of the most important elements in
the classification of fishes, their special arrangement and form will
be referred to in the account of the various families and genera.
Whilst not a few fishes are entirely edentulous, in others most of the
bones of the buccal cavity, or some of them, may be toothed, as the
bones of the jaws, the palatines, pterygoids, vomers, basisphenoid,
glossohyal, branchial arches, upper and lower pharyngeals. In others
teeth may be found fixed in some portion of the buccal membrane without
being supported by underlying bone or cartilage; or the teeth have
been developed in membrane overlying one of the dentigerous bones
mentioned, without having become anchylosed to the bone. When the tooth
is fixed to the bone the attachment has generally been effected by the
ossification of the bone of the tooth, but in some fishes a process of
the bone projects into the cavity of the tooth; in others the teeth are
implanted in alveoli. In these, again, frequently a process of bone
rises from the bottom, on which the tooth rests.
Many fishes, especially predatory fishes with long, lancet-shaped
teeth, have all or some of the teeth capable of being bent inwards
towards the mouth. Such “hinged” teeth resume at once the upright
position when pressure is removed from them. They are, however,
depressible in one direction only, thus offering no obstacle to the
ingress, but opposing the egress of prey. Mr. C. S. Tomes has shown
that the means by which this mechanism is worked are different in
different fishes; for whilst, in the _Pediculati_ and _Gadoids_ (Hake)
the elasticity resides solely in the tissue of the hinge (the tooth
being as resilient as ever after everything else is severed), in the
Pike the hinge is not in the least endowed with elasticity, but the
bundles of fibres proceeding from the interior of the dentine cap are
exceedingly elastic.
As regards texture the teeth of fishes show considerable variation.
The conical teeth of the Cyclostomes and the setiform teeth of many
Teleosteans consist of a horny albuminous substance. The principal
substance of the teeth of other fishes consists of dentine, with
numerous dividing and anastomosing tubercles, sometimes covered by
a stratum of unvascular dentine. An enamel-like substance has been
observed on the crown of the teeth of _Sargus_ and _Balistes_, and an
ossification of the capsule of their matrix covers the enamel with a
thin coating of cement. The teeth either possess a cavity in which the
matrix is received, or, more frequently, they are solid, in which case
vascular canals of the underlying bone are continued into the substance
of the tooth. In the teeth of some fishes numerous sets of canals and
tubes are so arranged that they do not anastomose with one another,
each set being surrounded by a layer of dentine and cement. These
apparently simple teeth are evidently composed of numerous small teeth,
and called _compound_ teeth.
The teeth may be, and generally are, very different as regards size or
form in the different parts of the mouth; they may be also different
according to the age or sex of the fish (_Raja_). The teeth may be few
in number and _isolated_, or placed in a single, double, or triple
_series_, _distant_ from one another or _closely set_; they may form
narrow or broad _bands_, or _patches_ of various forms. As regards
form, they may be cylindrical or conical, pointed, straight, or curved,
with or without an angular bent near their base; some are compressed
laterally or from the front backwards; the latter may be triangular
in shape, or truncated at the top like incisors of mammals; they may
have one apex (cusp) only, or be bi- or tri-lobate (bi- or tri-cuspid);
or have the margins denticulated or serrated. Compressed teeth may be
confluent, and form a cutting edge in both jaws, which assume the shape
of a parrot’s beak (Fig. 53). In some the apex is hooked or provided
with barbs. Again, some teeth are broad, with flat or convex surface,
like molar teeth. With regard to size, the finest teeth are like fine
flexible bristles, _ciliiform_ or _setiform_; or, if very short and
anchylosed to the bone, they appear only as inconspicuous asperities
of the bone. Very fine conical teeth arranged in a band are termed
_villiform_ teeth; when they are coarser, or mixed with coarser teeth,
they are _card-like_ (dents en rape or en cardes) (Fig. 54); molar-like
teeth of very small size are termed _granular_.
[Illustration: Fig. 53.--Jaws of Calliodon.]
In all fishes the teeth are constantly shed or renewed during the
whole course of their life. In fishes which have compound teeth, as
the Dipnoi, Chimæroids, Scari,[14] Gymnodonts, as well as in those
which have apparently permanent teeth, as in the saw of _Pristis_,
the detrition of the surface is made up by a constant growth of the
tooth from its base. When the teeth are implanted in alveoli, they
are generally succeeded by others in the vertical direction, but in
others they succeed one another, side by side. In the majority of
fishes the new tooth is not developed (as in reptiles and mammals) in
a diverticulum of the sack of its predecessor, but like this from the
free surface of the buccal membrane. Generally there are more than one
tooth growing, which are in various stages of development, and destined
to replace the one in function. This is very conspicuous in Sharks,
in which the whole phalanx of their numerous teeth is ever marching
slowly forwards (or in some backwards), in rotatory progress, over the
alveolar border of the jaw, the teeth being successively cast off after
having reached the outer margin, and fulfilled for a longer and shorter
period their special function.
[The richest materials for our knowledge of the teeth of fishes
are contained in _Owen’s_ “Odontography.” Lond. 1840. 8vo.]
[Illustration: Fig. 54.--Cardlike teeth of Plectropoma dentex,
with canines.]
The _intestinal tract_ is divided into four portions: œsophagus,
stomach, small and large intestine; two or more of these divisions
may coalesce in fishes and become indistinguishable. But it is
characteristic of the class that the urinary apertures are constantly
situated behind the termination of the intestinal tract.
In _Branchiostoma_ the whole intestinal tract is straight, and
coated with a ciliated mucous membrane. The wide pharynx passes into a
narrow œsophagus, this into a gastric cavity, the remainder being again
narrower and terminating in the anal aperture, which lies somewhat
to the left of the median line. The liver is represented by a green
coloured cœcal diverticulum of the stomachic dilatation. A mesenterium
is absent.
In the _Cyclostomi_ the intestinal tract is likewise straight, and
without clearly defined divisions; however, in _Petromyzon_ the
œsophagus shows numerous longitudinal folds, and the intestine proper
is provided with a single longitudinal fold. A mesentery, which is
present in the Myxinoids, is represented by a short median fold only,
by means of which the hindmost part of the intestine is fixed.
The _Palæichthyes_ show differences in the structure of their
intestinal tract as considerable as are found among the _Teleostei_,
but they have that in common that the absorbent surface of their
intestine is enlarged by the development of a spiral valve, evidence
of the presence of which in extinct Palæichthyes is still preserved in
the fossilised fæces or _coproliths_, so abundant in some of the older
strata.
In _Chondropterygians_ (Fig. 55) the stomach is divided into a cardiac
and pyloric portion, the former frequently terminating in a blind
sac, and the latter varying in length. The pyloric portion is bent
at its origin and end, and separated from the short duodenum (called
_Bursa entiana_ in these fishes) by a valve; the ductus hepaticus and
pancreaticus enter the duodenum. This is succeeded by the straight
intestine provided with the spiral valve, the coils of which may
be either longitudinal and wound vertically about the axis of the
intestine, as in _Carcharias_, _Galeocerdo_, _Thalassorhinus_, and
_Zygœna_, or they may be transverse to that axis, as in the other
genera. The number of gyrations in the latter case varies: there may be
as many as forty. The short rectum passes into a cloaca, which contains
also the orifices of the urogenital ducts. Only the commencement and
end of the intestinal tract are fixed by mesenterial folds.
In the _Holocephali_ and _Dipnoi_, the intestinal tract is short,
straight, and wide, without stomachic dilatation, a pyloric valve,
close to which the ductus choledochus enters, indicating the boundary
of the intestine proper (Fig. 57, _p_). The spiral valve is perfect,
and makes from three (_Chimæra_) to nine (_Ceratodus_) gyrations. A
cloaca is present, as in Chondropterygians. A mesentery fixing the
dorsal side of the intestine is absent.
[Illustration: Fig. 55.--Siphonal stomach and spiral valve of
Basking-Shark (Selache). (After Home and Owen.)
_a_, Œsophagus; _b_, Cardiac portion of stomach; _c_,
pyloric portion; _d_, pouch intermediate between stomach and
duodenum, with circular valves at both ends; _e_, Duodenum;
_f_, Valve of intestine; _g_, Ductus hepaticus; _h_, Spleen.]
The other _Ganoids_ resemble again more the Chondropterygians
in the structure of their intestinal tract. The stomach has always a
distinct pyloric portion, and has a still more complicated structure
in _Acipenser_. The duodenal portion receives the contents of
_Appendices pyloricæ_, which are confluent into a gland-like mass
in _Acipenser_, but separate in _Polyodon_, and numerous and
short in _Lepidosteus_, whilst _Polypterus_ possesses one
such appendage only. A spiral valve is developed in the Sturgeons and
Polypterus, but in _Amia_, in which the intestine performs several
convolutions, the four gyrations of the valve are situated far back
towards the end of the intestine. In _Lepidosteus_ the valve is
rudimentary, and indicated only by three raised lines crossing the
terminal portion of the intestine. In all these Ganoids the rectum has
a separate opening, without cloaca.
The structure of the intestinal tract of _Teleosteous_ fishes is
subject to so numerous modifications that we should go beyond the
limits of the present work if we would attempt to enter into details.
Great differences in this respect may be found even in groups of the
same natural families. Frequently the intestinal tract remains of
nearly the same width throughout its course, and only the entrance
of the various ducts serves as a guide for the distinction of its
divisions. An intestine of such uniform width may be straight and
short, as in _Scombresocidæ_, _Symbranchidæ_, or it may be more or less
convoluted and long, as in many _Cyprinidæ_, _Doradina_, etc. On the
whole, carnivorous fishes have a much shorter and simpler intestinal
tract than herbivorous.
In the majority of Teleosteans, however, œsophagus, stomach, duodenum,
small intestine and rectum, can be more or less distinctly, even
externally distinguished.
There are two predominant forms of the stomach, intermediate forms
being, however, numerous. In the first, the _siphonal_, it
presents the form of a bent tube or canal, one-half of the horse-shoe
being the cardiac, the other the pyloric portion. In the second, the
_cæcal_, the cardiac division is prolonged into a long descending
blind sac, the cardiac and pyloric openings of the stomach lying close
together (_Clupea_, _Scomber_, _Thynnus_, etc.)
[Illustration: Fig. 56.--Siphonal Stomach and Pyloric Appendages
of a Female Salmon, 3⅓ feet long. _a a a_, Pyloric appendages;
_ch_, ductus choledochus; _oe_, œsophagus; _st_, lower end
of stomach; _p_, pyloric region; _i_, ascending; and _í’_,
descending portion of intestine.]
The duodenum receives always the hepatic and pancreatic secretions,
and, besides, those of the appendices pyloricæ, which, in varying
numbers (from 1 to 200), are of very common occurrence in Teleosteans
(Fig. 56). They vary also in length and width, and whilst the
narrowest serve only as secretory organs, the widest are frequently
found filled with the same contents as the intestine. When few in
number, each opens by a separate duct into the duodenum; when their
number is greater two or more coalesce into a common duct; in the
latter case the appendages cease to be free, and are connected with one
another by a more or less firm tissue.
Cœcal appendages at the end of the intestinal tract are of exceedingly
rare occurrence in fishes (_Box_). There is no _cloaca_ in Teleosteans.
In the majority of Teleosteous fishes the _vent_ is situated on the
boundary between trunk and tail, behind the ventral fins. In a few it
lies farther backwards, not far from the caudal fin; more frequently
it is advanced forwards, under the middle of the abdomen or to the
scapular arch. In two fishes, _Aphredoderus_ and _Amblyopsis_, it lies
before the pectoral fins.
A peritoneum envelops all the divisions of the intestinal tract within
the abdominal cavity. A broad, well-developed _omentum_ has hitherto
been found in _Gobiesox cephalus_ only.
_Liver_.--The existence of a liver in _Branchiostoma_ as a long
diverticulum of the intestine has been mentioned above. In the
Myxinoids the liver is divided into two glandular bodies, an anterior
rounded smaller one, and a posterior larger one of an elongate shape.
The gall-bladder lies between both, and receives a cystic duct from
each of them. In the other fishes the proportionally large liver is a
single large gland, from which only now and then small portions are
found to be detached. It is either simple, or with a right and left
lobe, or with a third lobe in the middle; each lobe may have incisions
or subdivisions, which, however, are very inconstant. The liver of
fishes is distinguished by the great quantity of fluid fat (oil) which
it contains. The gall-bladder is but rarely absent, and attached to
the right lobe, or towards the centre; however, in some fishes it
is detached from the liver and connected with it by the cystic duct
only. The bile may be conveyed by one or more hepatic ducts into a
common duct which is continued towards the gall-bladder as _ductus
cysticus_, and towards the duodenum as _ductus choledochus_; or some
of the hepatic ducts enter directly the gall-bladder, or directly
the duodenum, without communicating with the common duct. Individual
variations in this respect are of common occurrence.
A _pancreas_ has been found hitherto in all Chondropterygians,
Acipenser, and many Teleosteans. In the first it is a glandular mass
of considerable size behind the stomach, close to the spleen; its duct
leads into the duodenum. In the Sturgeons the pancreas is attached to
the duodenum, and opens close to the ductus choledochus. In _Silurus
glanis_ it is very large, and the ductus choledochus passes through
its substance; it is smaller in _Belone_ and _Pleuronectes_, and
situated in the mesentery; its duct accompanies the terminal portion
of the ductus choledochus. In the Salmon, which possesses a large
lobed pancreas, the duct is so intimately connected with the ductus
choledochus that both appear externally as a single duct only.
The _spleen_, which is substantially a lymphatic gland, may be
mentioned here, as it is constantly situated in the immediate vicinity
of the stomach, generally near its cardiac portion. With the exception
of _Branchiostoma_, it is found in all fishes, and appears as a rounded
or oblong organ of dark-red colour. In the Sharks frequently one or
more smaller pieces are detached from the principal body. In the Dipnoi
a thin layer of a very soft substance of brownish-black colour below
the mucous membrane of the stomach and upper part of the intestine has
been regarded as the homologue of the spleen (Fig. 57, _m_). In most
_Teleostei_ the spleen is undivided, and appended by its vessels and
a fold of the peritoneum to the pyloric bend of the stomach or the
beginning of the intestine.
[Illustration: Fig. 57.--Upper part of Intestine of Ceratodus.
The anterior wall of the intestine is opened, the liver (_c_) and
gall-bladder (_e_) being drawn forward. A slit is made at _n_,
through which part of the next compartment of the spirally wound
intestine may be seen.
_é_, Mouth of ductus choledochus; _f_, stomach; _i_, adipose
agglomeration; _l_, first compartment of intestinal spire; _m_,
spleen; _oe_, lower part of œsophagus, opened; _p_, double
pyloric fold; _q_ _q_, glandular patches.]