Page 355 - My FlipBook
P. 355
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TEETH OF THE VERTEBRATA. 365
I. Skull undeveloped, witli the notochord persistent and extending to the anterior
end of the liead. Brain not distinctly ditlerentiated. Heart none.
Leptocardii.
II. Skull more or less developed, with the notochord not continued forward beyond
the pituitary body. Brain differentiated and distinctly developed. Heart
developed and divided at least into auricle and ventricle.
A. Skull imperfectly developed, with no lower jaw. Paired fins undeveloped,
with no shouldei'-girdle nor pelvic elements. Gills purse-shaped.
Maksipobranchii.
B. Skull well developed, with a lower jaw. Paired fins developed (sometimes
absent through atropiiy), and with shoulder-girdle (lyrii'orm or furcula-
shaped, curved forward, and with its respective sides connected below), and
with pelvic elements. Gills not purse-shaped Lyrifera.
a. Skull witiiout membrane bones ("a rudimental opercular bone" in Cliiincera)
gills not free, the branchial openings slit-like, usually several in number
exoskeleton placoid, sometimes obsolete ; eggs few and large.
Elasmobranchii.
b. Skull with membrane bones; gills free; branchial openings a single slit on each
side, sometimes confluent ; exoskeleton various, not piacoid ; eggs compara-
tively small and numerous Pisces.
Tlie first of these classes, Leptoeardii, includes a few small fish-like
animals, such as the well-known amphioxus or lancelet occurring on our
coast, in which no skull exists. They are in many ways most remark-
able f )rms, being the most primitive of all vertebrates, but as they are
devoid of teeth, this class can be dismissed without further consideration.
The next, Marslpohranch'ii, embraces the lampreys, whose "horny teeth"
have already been alluded to. The relationship as well as examples of
each order of the remaining two classes is expressed in the subjoined
table (p. 366), which is compiled from Dr. Gill's papers on the
classification of fishes.
The Accessot'i/ Organs.—A consideration of these organs necessarily
involves not only a study of the bones and cartilages taking share in
the boundary of tiie oral cavity, but of all bones and cartilages in connec-
tion with which teeth are developed. It would likewise properly include
a mention of the muscles which move these parts, together with the vas-
cular and nervous supply ; but o^viug to their great range of variation,
as well as the limited space at my disposal, these latter will not be con-
sidered. This, in my juilgment, is best accomplished by describing the
normal arrangement in some typical fish and comparing all others with
it. For this purpose a gadoid fish, or one of the cod tribe, is most suit-
able, since it exhibits the structure which obtains in a large majority of
ichthyic forms.
If a well-cleaned skull be examined, it will be seen to consist, in the
first place, of a cranium or brain-box, or that part which remains intact
after the skull has been boiled or macerated a sufficient length of time
to cause the soft parts to disappear and the arches and appendages to
become disarticulated. This contains the brain, and becomes continuous
at its lower back part with the vertebrae or axial pieces of the body skele-
ton into wliicli the spinal cord jiasses. Suspended from either side of its
posterior portion there is a chain of bones which extends down beneath
the throat and bears the pectoral fins ; this is known as the shoulder-gir-
dle or scapular arch (see Fig. 190).
A short distance in front of this, or at a point about midway between
the root of the scapular arch and the eye-socket, another arch springs
TEETH OF THE VERTEBRATA. 365
I. Skull undeveloped, witli the notochord persistent and extending to the anterior
end of the liead. Brain not distinctly ditlerentiated. Heart none.
Leptocardii.
II. Skull more or less developed, with the notochord not continued forward beyond
the pituitary body. Brain differentiated and distinctly developed. Heart
developed and divided at least into auricle and ventricle.
A. Skull imperfectly developed, with no lower jaw. Paired fins undeveloped,
with no shouldei'-girdle nor pelvic elements. Gills purse-shaped.
Maksipobranchii.
B. Skull well developed, with a lower jaw. Paired fins developed (sometimes
absent through atropiiy), and with shoulder-girdle (lyrii'orm or furcula-
shaped, curved forward, and with its respective sides connected below), and
with pelvic elements. Gills not purse-shaped Lyrifera.
a. Skull witiiout membrane bones ("a rudimental opercular bone" in Cliiincera)
gills not free, the branchial openings slit-like, usually several in number
exoskeleton placoid, sometimes obsolete ; eggs few and large.
Elasmobranchii.
b. Skull with membrane bones; gills free; branchial openings a single slit on each
side, sometimes confluent ; exoskeleton various, not piacoid ; eggs compara-
tively small and numerous Pisces.
Tlie first of these classes, Leptoeardii, includes a few small fish-like
animals, such as the well-known amphioxus or lancelet occurring on our
coast, in which no skull exists. They are in many ways most remark-
able f )rms, being the most primitive of all vertebrates, but as they are
devoid of teeth, this class can be dismissed without further consideration.
The next, Marslpohranch'ii, embraces the lampreys, whose "horny teeth"
have already been alluded to. The relationship as well as examples of
each order of the remaining two classes is expressed in the subjoined
table (p. 366), which is compiled from Dr. Gill's papers on the
classification of fishes.
The Accessot'i/ Organs.—A consideration of these organs necessarily
involves not only a study of the bones and cartilages taking share in
the boundary of tiie oral cavity, but of all bones and cartilages in connec-
tion with which teeth are developed. It would likewise properly include
a mention of the muscles which move these parts, together with the vas-
cular and nervous supply ; but o^viug to their great range of variation,
as well as the limited space at my disposal, these latter will not be con-
sidered. This, in my juilgment, is best accomplished by describing the
normal arrangement in some typical fish and comparing all others with
it. For this purpose a gadoid fish, or one of the cod tribe, is most suit-
able, since it exhibits the structure which obtains in a large majority of
ichthyic forms.
If a well-cleaned skull be examined, it will be seen to consist, in the
first place, of a cranium or brain-box, or that part which remains intact
after the skull has been boiled or macerated a sufficient length of time
to cause the soft parts to disappear and the arches and appendages to
become disarticulated. This contains the brain, and becomes continuous
at its lower back part with the vertebrae or axial pieces of the body skele-
ton into wliicli the spinal cord jiasses. Suspended from either side of its
posterior portion there is a chain of bones which extends down beneath
the throat and bears the pectoral fins ; this is known as the shoulder-gir-
dle or scapular arch (see Fig. 190).
A short distance in front of this, or at a point about midway between
the root of the scapular arch and the eye-socket, another arch springs
