The spindle-shaped body tapers from near the middle toward both head and
tail; the head is flattened on both the dorsal and ventral sides, while
the remainder of the body is nearly round, with a lateral compression
which is not pronounced except in the caudal portion.
The general color of the back and sides is gray; darkest above, where
the skin is spotted with scattered, small, round, light spots. The color
of the upper parts shades into the yellow white of the ventral surface.
Can head, trunk, and tail regions be distinguished? If so, what
characters determine the extent of each?
A little above the middle of the side of the body is the _lateral line_,
(distinguished partly by color, partly by being slightly elevated),
which extends from the back of the head to the tail. Cut through the
skin across the lateral line at several points along the body and notice
the canal which lies in the dermis under the lateral line. This is the
_lateral line canal_, which opens to the surface by numerous pores (too
minute to be seen), and contains a series of special sense organs along
its dorsal and inner surfaces. Near the base of the caudal fin the
lateral line canal passes into a groove which continues the lateral line
to within a short distance of the edge of the fin. The lateral line
canal in its development begins as a groove along the side of the body
which becomes closed by the fusion of its edge except in this terminal
portion.
The open groove does not appear in Eugaleus.
In the midline of the body are two triangular _dorsal fins_, each
attached to the body for about half its length. The basal portion of
each is thick and muscular, and contains supporting cartilages embedded
in the muscles. The remainder of the fin is flexible and
semi-transparent, horny fin-rays being faintly visible between the
layers of skin. In front of each fin is a strong spine which seems to
serve both as a cutwater and a weapon of defense. (Eugaleus has no
spines.)
The broad, paired, _pectoral fins_, having the general characteristics
of the dorsals, spring from the ventral edges of the body just back of
the head. A hard bar of cartilage connecting the bases of the pectoral
fins can be felt through the skin of the ventral surface of the body.
This is the ventral part of the _pectoral girdle_.
Farther back, and also on the ventral surface, are the paired _pelvic
fins_. The pelvic girdle can be felt through the skin between the bases
of these fins. If the specimen is a male, it will have a fingerlike
process projecting backward from the base and along the inner side of
each pelvic fin. These organs, which attain a considerable size in
adults, are modified portions of the pelvic fin used as copulatory
organs. They are named variously _claspers_, myxopterygia, or
pterygopodia. A groove runs along the dorsal side of the clasper from
the tip to near the base, where it opens into a long sac (_glandula
pterygopodia_) extending some distance in front of the pelvic fins just
within the skin of the ventral surface. By feeling a cartilaginous axis
of the clasper can be discovered, which extends to the tip of the organ.
On the dorsal surface and near the end of the clasper is a sharp grooved
spine on the outer side, and a strongly recurved hook on the inner side,
both almost hidden by a flap projecting from the inner edge of the
groove. This flap is stiffened by a series of small cartilages similar
to the radial cartilages in the bases of the fins.
The _caudal fin_ is asymmetrical, extending along the dorsal and ventral
edges of the posterior end of the body. Observe the upward bend of the
vertebral column which occurs in the tail; it is this character which
marks the heterocercal type of tail.
Eugaleus has a median ventral, or _anal_, fin a short distance anterior
to the caudal fin.
The _mouth_ is a broad transverse slit upon the ventral surface of the
head. The cartilaginous jaws can be seen and felt just within the mouth.
Both upper and lower jaws are armed with rows of flat, sharply pointed
teeth. Study their arrangement and approximate number. Jaws of
previously dissected specimens should be examined under a dissection
microscope. The exact form and arrangement of the functional teeth can
then be ascertained easily, and an examination of the inner surface of
either jaw will disclose several rows of developing teeth. As the young
teeth develop they move, a row at a time, into position on the edge of
the jaw; the oldest teeth, occupying the outer row, are shed at about
the same time.
The upper jaw is partly overhung by a lip-like fold of skin. At each
side of the mouth is a pocket, directed obliquely, having no
communication with the mouth. These _labial pockets_ provide places for
the _labial cartilages_ (which can be felt along the medial edges) when
the mouth is closed, and also afford freedom of motion to the mandible.
Cut along the inner edge of the labial pocket and expose the cartilages
for examination. (The labial pocket of Eugaleus is much smaller, and in
front of the corner of the mouth rather than lateral to it. The two
cartilages are completely separated from each other, the posterior one
scarcely reaching the pocket.)
In front of the mouth are the _nostrils_, their apertures apparently
divided by projecting flaps of the anterior margin. Explore the cavity
of the nostril with a probe to get a good idea of its size and form.
Between the pelvic fins is the _cloaca_, a large depression into which
open the alimentary canal, the excretory and genital ducts, and the
abdominal pores. The opening of the alimentary canal, the _anus_, is at
the anterior end of the cloaca. In preserved specimens part of the
intestine is frequently everted through the anus. A large fleshy
process, bearing a pore at its tip, projects from the dorsal wall of the
cloaca. In the male this is the _uro-genital papilla_; in the female the
_urinary papilla_. In the female a _genital pore_, the opening of the
oviduct, is found on either side of the papilla. An _abdominal pore_,
leading into the abdominal cavity, is found on each side of the cloaca
at the posterior margin. These are frequently closed in young specimens.
The cloaca of Eugaleus has a comparatively small opening upon the
ventral surface, which must be enlarged before the parts described can
be seen well.
The laterally placed _eyes_ are without lids; observe the considerable
difference in the amount of curvature of the dorsal and ventral margins
of the eye.
In Eugaleus there is a fold of skin stretched across the lower part of
the eye which serves as an eye-lid, and corresponds to the so-called
“third eye-lid” or nictitating membrane of other vertebrates.
On each side of the neck are five vertical _gill-clefts_, each leading
into a large _gill pouch_ which communicates with the pharynx by an
internal opening. Pass a probe through a gill-cleft into the mouth.
Back of each eye is a small aperture, the _spiracle_; explore this
cavity with a probe. The spiracle is to be considered a gill-cleft moved
forward upon the head and largely, though not entirely, deprived of its
respiratory functions.
In the center of the dorsal surface of the head, between the spiracles,
are two pores, the _external openings of the endolymphatic ducts_ which
communicate with the internal ear. Large numbers of smaller pores can be
found on all surfaces of the head, some in groups, some arranged
linearly, many scattered. Most of those arranged in lines lead into the
sensory canal system which continues from the lateral line canal upon
the head, while the majority of the others belong to a separate type of
sense organs, the _ampullae of Lorenzini_.
Make a cut encircling the pores of the endolymphatic ducts and close to
them. Do not remove this piece of skin. From it make a median incision
forward to the tip of the snout and back as far as the level of the
first gill slit. Starting at this incision work the skin off from the
tissues beneath it. This must be a careful, close dissection. When the
lateral line is reached it will be seen that the lateral line canal is
continued upon the head and is joined by several others. By looking
through the loosened skin toward the light the pores can be seen which
lead from the canals to the surface. In the hollow of the skull in front
of and above the eye is a large group of tubules which open through the
pores so conspicuous at this point. At the internal end of each tubule
is a slight enlargement, of denser tissue, with which a delicate nerve
strand can often be seen connected. The nerve strands can be traced to a
large nerve passing above the eye and distributed to the snout. These
tubules are the _ampullae of Lorenzini_. This group of ampullae, which
may be called the dorsal group, is quite definitely demarked. It will be
noted that the inner ends of the ampullae are grouped in a much smaller
area than their pores.
Between the spiracle and the first gill slit will be found a lateral
group of similar organs. Notice the arrangement of their tubules and
pores. Under the snout are two groups of ampullae on each side of the
midline. The inner ventral group is separated from the outer ventral
group by the lateral bar of the rostral cartilage. Some of the tubules
of the outer ventral group will be found to extend to pores situated at
the sides of and behind the mouth.
An adult fish possesses from 1200 to 1900 ampullae of Lorenzini. Their
function is not well understood, but it has been suggested that they are
organs responsive to stimuli of pressure, either of currents or water,
or resulting from depth, or even of deep tones.
The _system of sensory canals_ consists of the following members on each
side of the head:—
A _supra-orbital_, passing above the eye to the end of the snout and
bending back on the ventral surface to join the infra-orbital.
An _infra-orbital_, which branches off from the supra-orbital and passes
ventrad between the eye and the spiracle, then turns forward along the
ventral margin of the orbit, and finally bends toward the midline and
extends to the tip of the snout.
A _hyomandibular_, which leaves the infra-orbital below the eye and runs
back beyond the angle of the mouth.
A short _mandibular_, on the mandible close to the angle of the mouth,
which is not connected with the other canals.
The canal systems of the right and left sides are connected by a
_supratemporal_ canal just behind the endolymphatic pores, and often by
an anastomosis of the infra-orbital canals in front of the mouth.
On some specimens two crescentic rows of pores can be found between the
bases of the pectoral fins, which represent a third type of sensory
organ, the _pit organs_, closely related genetically to the sensory
canals. Two longer lines of pit organs, (the _mandibular pit organs_),
will be found a short distance behind the mouth. Similar pit organs are
found in front of the endolymphatic pores, and above the anterior part
of the lateral line.
Except for a few small areas the entire surface of the body is covered
with small, sharp-pointed denticles (_placoid scales_). Each consists of
a diamond-shaped basal plate embedded in the dermis, from which projects
a leaf-like, backward directed spine. A piece of skin should be removed
and examined under a low magnification. The dermis is so dense and
pigmented that the basal plate is not easily studied without further
manipulation. For this purpose boil a piece of skin in 5% caustic potash
solution until it is softened, but not till it disintegrates. Then clear
it in glycerine. Examine the individual denticles under a higher
magnification. The denticle consists of dentine, the spine being of a
much denser structure than the base. The teeth and the large spines of
the fins and claspers are also composed of dentine and may be considered
as modified placoid scales. Denticles, teeth, and spines are covered
with a shiny, enamel-like layer which, however, does not appear to be
true enamel such as covers the teeth of higher vertebrates. The shape of
the scales and their closeness vary on different regions of the body,
and there are certain regions entirely free from them, namely, back of
the dorsal, pectoral and pelvic fins, the medio-dorsal surfaces of the
claspers, inside the upper lip and the labial pockets.