On the Osteology and Development of Syng- nathus Peckianus

28 Pages·2006·1.21 MB·English

OSTEOLOGY ETC., OF SYNGNATHUS PECKIAFOS. 623
On the Osteology and Development of Syng-
nathus Peckianus (Storer).
By
a. Playfair McMurrich, M.A.,
Professor in the Ontario Agricultural College, Guelph, Canada.
With Plates XLII and XLIII.
IT is now some time since I began the study of Syngna-
thus, recognising the fact that little, if anything, had been
done towards elucidating many points in the anatomy and de-
velopment of the grotesque group of the Lophobranchs. Other
matters have prevented as rapid work, and as close application
to the subject in hand, as would have been wished for, and
even now the work is exceedingly incomplete. Nevertheless
it has seemed to me to be wise to publish the observations as
far as completed, inasmuch as I have been able to throw some
light on certain points, and to correct some mistakes made by
authors who have preceded me.
The material for study was obtained at Beaufort, N.C.,
where it was quite abundant among the seaweed near the
shore. The presence of the brood-pouch on the under surface
of the postabdominal region of the male renders the collection
of young stages comparatively easy, but the intermediate stages
between the newly hatched young and the adult were less
easily obtainable, and I have been unable to bridge over this
gap in my observations. The material under observation may
be divided into five stages, the relative characters of which may
be described as follows:
624 J. PLAYFAIR MOMUREIOH.
A. Length 3—4 mm.: cartilages not quite fully formed.
B. Length 6—7 mm.; cartilages fully formed; considerable
amount of yolk still present.
c. Length 8—9 mm.; yolk-sac very much diminished in
size.
D. Length 10—11 mm.; yolk completely absorbed; ready to
leave the brood-pouch.
E. Adult.
I. DEVELOPMENT AND STRUCTURE OF THE CRANIUM.
In Stage A (PI. XLII, fig. 1) the head is round and small,
with the lower jaw bent up, and closely applied to the under
and anterior surface of the cranium, there being as yet no dif-
ferentiation of the upper jaw and snout. Below the medulla
oblongata (M. 0.) the notochord (N.C.) bends abruptly
downwards, and becomes ensheathed on either side by the
parachordal cartilages, which are continued forward to unite
with the tTabeculse cranii (Tr. Cr.), the extremity of which
is somewhat bent upwards, and as yet, as it were, within the
cranium, reaching no farther forwards than a point between
the eyes. The auditory capsules (Au.) are formed in carti-
lage, and are apparently united to the parachordals, but possess
no rudiment of the semicircular canals, though two otoliths are
plainly visible.
In Stage B (fig. 2) the snout makes its appearance, but it is
as yet bent upwards, and lies closely applied to the front of the
cranium, owing to the upward flexion of the trabeculse men-
tioned above, which has increased considerably. At their co-
alesced extremities, and articulating with their sides, is on
either side a cartilage (E. Pa.), whose signification will be
discussed when treating of the visceral arches. When the
skull in this stage is viewed from above (fig. 4), the nares (01.)
may be seen lying on the upper surface of the snout, slightly
anterior to the eyes. They have a thickened margin, and be-
hind them, extending down from the centre towards the sides
of the snout or rostrum, are two cartilaginous bars (Na. C),
OSTEOLOGY ETC., OF STNGNATHUS PECKIANUS. 625
while above these is a cartilaginous plate (Tg. Cr.) extending
upwards to between the eyes, and produced to a point between
the nares, forming the tegnien cranii. This and the two
nasal rods have apparently no connection at present with the
coalesced trabeculse, but are apparently independent cartilages:
the tissue lying between the nares and the extremity of the
trabeculse is somewhat fibrous. The auditory region is now
more fully developed, and the semicircular canals are repre-
sented by fibrous bands. The parachordals posteriorly have
extended upwards and around the nervous cord to form the
occipital region of the skull.
Stages c and D are very slightly different. They present au
increase in size from Stage B, and the fibrous semicircular
canals have become cartilaginous. The snout has increased
somewhat in length in c, and still more in Stage D, the growth
taking place at first by an increase iu length of the horizontal
portion of the cartilaginous " rostral plate," as the elongated coal-
esced trabeculse may be called, and, latterly, by a straightening
out of its bent up end, which even in Stage c (fig. 5) forms a
right angle with the horizontal portion. The posterior region
of the skull is almost completely encased in cartilage, its very
summit only being of dense membrane ; more anteriorly the
membrane becomes thinner, and extends further down the
sides of the skull.
At the base of the skull, and extending from its posterior
portion forward for a considerable distance in the median line,
is a dense membrane (fig. 15, Pa. S.), identical in appearance
with that roofing in the cranial cavity, and also with the mem-
brane surrounding the notochord at this stage: this is the
commencement of the formation of a bone, which from its
position must be the parasphenoid. At the sides of the
vertex of the anterior portion of the cranium proper, and ex-
tending back nearly to its posterior portion, are two similar
condensations of tissue (Fr. S.). They consist, as is seen on
section, of plates lying in the connective tissue enclosing the
cranial cavity, one on either side, and at the centre of each
plate and perpendicular to it a ridge passes along its entire
626 J. PLAY.FAIR MOMURRICH.
length, projecting out into the integument. Still more poste-
riorly only one such plate of membrane is seen, which occupies
a median position at the vertex. These appear to be the
membranous rudiments of the frontals and dermo-supra-
occipital.
The Adult.—In the adult cranium the cartilage persists to a
gveat extent, although surrounded almost completely by bone.
No enchondroses, as in the higher vertebrates, appear; ectostoses
and parostoses form the cranium. One point noticeable at the
first glance is the elongation of the occipital regions, and the
oompactness of the region immediately behind the orbits;
separating these two portions there is a membranous space,
closed in by the parasphenoid.
The occipital region ossifies below as the basi-occiptal (fig.
9, B. O.). Posteriorly this is round, forming an articular facet
for the first vertebra, but anteriorly it becomes flattened out,
and expanded into a thin plate—being, in fact, fan-shaped.
Posteriorly, on section, the rapidly diminishing notochord is
plainly visible, and on either side of this are the parachordal
cartilages. More anteriorly the chorda does not appear, and
the parachordals unite to form a single plate. Below it is
deeply grooved for the reception of the parasphenoid, which
underlies it, and is almost enclosed by it. Laterally it articu-
lates with the exoccipitals, and more anteriorly with, the
ptero tics.
The exoccipitals (Ex. O.)are well-developed bones, forming
the postero-lateral floor of the skull, and extending somewhat
upwards upon its sides to articulate with the epiotics, and in.
front with the pterotics. Above them on either side is a
parosteal bone (fig. 8, S. Tp.), upon whose homologies I am
undecided. I am inclined, however, to consider it homologous,
to a certain extent at any rate, with the supra-temporal of
Amia.
Above, at the vertex of the skull, is the supra-occipital
(S. O.), a large bone, extending forward nearly to the sphenotics.
It appears to consist of two portions; (1) a parostosis, which
may be termed the dermo-supra-occipital, aud which de-
OSTEOLOGY, ETC., OF STNGNATHTJS PECKIANUS. 627
velops apparently in a manner similar to what occurs in the
placoid scales of the body, from the membranous plate already
spoken of; and (2) an ectosteal portion, which may be termed
the auto-supra-occipital. The supra-occipital is somewhat
narrowed in front, projecting between the posterior portions of
the frontals.
Laterally from this bone we have on either side the parie-
tal s (Pa.), small bones which do not meet in a sagittal suture,
but are separated throughout their entire length by the supra-
occipitals, as in the pike and the salmon. Anteriorly they
articulate with the frontals. The name dermo-epiotics,
which Bridge has suggested for these bones in Amia,1 is here
very applicable, for they directly overlie the epiotics, appearing
like a parostosis formed upon these bones.
The pterotics (Pt. 0.) extend far forward to a point just
behind the sphenotics, and form, along with the pro-otics and
sphenotics, the articulating surface for the hyomandibular.
They descend pretty well to the base of the skull, their poste-
rior parts overlapping the anterior lateral portion of the
basi-occipital, and articulating with the exoccipitals. They
form the lateral boundaries of the membranous space in front
of that bone. Anteriorly they articulate with the pro-otics.
These bones (fig. 9, Pr. O.) bound the membranous space in
front, and extend forward nearly to the anterior limit of the
postorbital region of the cranium. They extend only slightly
upwards on the sides of the skull to articulate with the pterotics
and sphenotics, and unite across the middle line of the skull
anteriorly, but are separated posteriorly, leaving a space closed
in only by the parasphenoid. At the sides in this region are
the sphenotics (Sp. O.). There seem to be no distinct ali-
sphenoids, the part of the skull having the usual relations of
these bones being ossified by the pro-otics. There is apparently
also no basisphenoid.
The frontals (fig. 8, Fr.) are membrane bones of compara-
tively large size, extending from the parietals posteriorly to
1 T. W. Bridge, " The Cranial Osteology of Amia cal va," J'ourn. of Anat-
and Pliys.,' vol. xi, 1877.
628 J. FLAYFAIB, MCMUREICH.
slightly in front of the ectethraoids anteriorly. They are some-
what club-shaped, broadening out posteriorly. On viewing
them from the surface they appear to be unsymmetrical, one
forming a projection which fits into a corresponding indentation
on the other. This want of symmetry is apparent rather than
real, for a section (fig. 11, Fr.) shows it to be caused by an
overlapping, the portion of bone overlapped being equal to that
which overlaps it. Opposite the sphenotics, from the inner
surface of each frontal, a process passes down, which articulates
with the front edge of the ascending or pro-otic process of the
parasphenoid.
The parasphenoid (fig. 9, Pas.) is a long parostosis, ex-
tending from the basi-occipital nearly to the anterior extremity
of the cranium on its under surface. Behind it is round, and
almost enclosed in the basi-occipital, lying in a deep groove in
that bone. More anteriorly it widens out to form a partial
floor for the membranous space in front of the basi-occipital, the
cartilaginous trabeculse lying immediately above it. At the
anterior limit of the postorbital region of the skull (fig. 10) it
sends up on either side a process, somewhat triangular in shape,
when viewed laterally. These articulate with the pro-otics, and
along their anterior edges with the descending processes of the
frontals. These processes may be termed the ascending or
pro-otic processes of the parasphenoid. The space bounded
laterally by these two bars, above by the frontals, and below
by the parasphenoid, serves for the passage of the orbital
muscles and nerves. Anterior to this, in the orbital region, the
parasphenoid becomes rectangular, and finally triangular (fig.
11, Pa. S.); the apex being directed upwards, having attached
to it the lower edge of the interorbital membrane. In front of
the orbit it articulates on either side with an ectethmoid, and
still more anteriorly (fig. 12) becomes convex, being deeply
grooved on the under surface, in which groove lies the vomer
(Vo.), almost enclosed, and presenting a similar appearance to
the parasphenoid when lying in the groove in the basi-occipital.
There is no orbitosphenoid, the passage for the orbital
muscles and nerves being very large, appearing, in fact, almost
OSTEOLOGY, ETC., OP STNGJSTATHUS PECKIANUS. 629
as if the whole anterior wall of the cranial cavity were wanting.
The in terorbital septum (fig. 11,1. S.) is merely membranous,
as in the Siluroids and Cyprinoids.
Immediately in front of the orbits, on either side, is an.
ectethmoid (fig. 8, Ect. E.), the ossification of the preorbital
process of the ethmoidal cartilage. They extend down the sides
of that cartilage, and articulate below with the parasphenoid,
and above with the frontals.
At this point or slightly anterior the frontals terminate, the
remainder of the rostrum being formed by the ethmoid, with,
the vomer lying along its under surface. There appear to be
no membrane bones occupying the position of the nasals of most
fishes.
The ethmoid (fig. 12, Eth.) consists posteriorly largply of
cartilage surrounded by a certain amount of ectosteal bone.
In the cartilage, on either side, is a canal, in which the olfac-
tory nerves and vessels run, passing to the olfactory capsules,
which form deep indentations in the sides of the cartilage.
Immediately in front of the orbits the cartilage appears to con-
sist of two portions, an upper, originally the tegmeu cranii, and
a lower, the coalesced trabeculee, and between these the olfac-
tory nerves and vessels at first run. This distinctness of the
two parts obtains, however, only for a short distance, the two
halves soon uniting and becoming indistinguishable, the olfac-
tory structures becoming enclosed in a canal. In front of the
olfactory organs the cartilaginous part of the ethmoid rapidly
diminishes, there being a nearly corresponding increase in bone.
In this a canal appears, which passes towards the surface as
one traces it forwards. I take this to be the continuation of
the main slime canals, which also traverses the frontals. Ante-
riorly the ethmoid becomes thinner, but remains nearly of the
same breadth, and its cartilaginous portion entirely disappears j
still more anteriorly (fig. 13) it becomes almost scale-like, the
vomer (Vo.), which hitherto has been round and small, now
becoming larger and triangular, and forming the greater part
of the thickness of the rostrum in this region, and this relation
persists to its extremity.
VOL. XXIII NEW SEH. T T
G30 J. PLAYFATR MCMURRTCff.
Summary.—The most noticeable feature in the configura,
tion of the embryonic cranium is the bending up of the facial
portion against the front of the skull, this being due to the
bending up of the coalesced trabeculae cranii at their extremity.
As development proceeds this bending up does not diminish-
but the mouth becomes carried forward by the growth of the
horizontal portion of the trabecula?, and it is not until Stage D
is arrived at that the elongation of the snout is dependent upoa
the straightening out of the rostral cartilage.
In the adult the first thing to be noticed is the forward
extension of the occipital region and the compactness of the pro-
otic region. The absence of any cartilagiuous sphenoid bone,
the wide opening for the passage of the orbital muscles and
nerves consequent upon this, the absence of an osseous inter-
orbital septum as in the Siluroids arid Cyprinoids, the want of
nasals, and the structure of the ethmoids, are also points of
considerable importance.
II. THE VISCERAL SKELETON.
In the youngest stage observed (A) most of the visceral
arches were apparently fully formed, and consisted of seven
cartilaginous bars, some, however, being more differentiated
than others, the first three already showing a specialisation
into their future parts.
The first postoral or mandibular arch consists, on either
side, of a well-developed mandibular portion (fig. 1, Mck.),
bounding the gape below, extending forwards and upwards,
and curving slightly inwards towards its fellow of the opposite
side. Articulating with the proximal extremity of each of
these Meckelian cartilages is a single rod-shaped portion
(Ft. Qu.), extending backwards and slightly upwards, but
lying quite free in the tissue of the prajvertebral portion of the
skull, except for the articulation with Meckel's cartilage.
Subsequent development shows this to be the rudiment of the
pterygo-quadrate portion of the jaw, and hence it may be
denominated the pterygo-quadrate cartilage.
The second postoral or hyomandibular arch is represented by
OSTEOLOGY, ETC., OP SYNGNATHTJS PECKIANUS. 631
a curved rod (H. M.), extending from the skull downwards and
forwards towards the proximal extremity of Meckel'a cartilage*
with which it appears to articulate. Anteriorly it lies below,
and almost parallel to, the pterygo-quadrate cartilage, and
becomes somewhat broader posteriorly. It presents as yet no
differentiation into the hyomandibular and symplectic portions,
to which this segment of the arch corresponds, nor in fact does
it so separate afterwards, the line of division between the two
being indicated merely by the articulation of the second seg-
ment of the arch, i. e. hyoid portion. This consists at present
of a rod lying below, and inclined towards, the upper cartilage,
which it meets a little in front of the dilated portion.
The remaining four arches constitute the branchial arches
(Brs.), present at this early stage to the same number as in
the adult. No azygos, median hyoid cartilage could be de-
tected at this stage, nor is there a median branchial cartilage.
In Stage B (figs. 2 and 3) the mandible has become bent up
upon the skull, and the pterygo-quadrate cartilage retains the
same characters as in the preceding stage. The hyomandibular
bas, however, undergone some modifications. It has become
distinctly angulated, the symplectic portion (Sym.) extending
forward parallel with the axis of the skull, while the hyoman-
dibular (H. M.) moiety is bent up almost at right angles to it,
slightly posterior to the eye, and articulates with the lateral
under-surface of the auditory capsule. Near the anterior
border of this portion is an oval foramen, and at the angle of
junction with the symplectic a separate cartilaginous centre
appears, uniting the hyoid to the hyomandibular portion of the
arch, and representing the interhyal. The hyoid arch (Hy.)
extends from this downwards towards the median line, and
represents the combined ceratohyalsand hypohyals. About
half way from the extremity of each portion of it is a process
extending upwards towards the symplectic, which is notched
to receive it. A median hyoid rod (Gr. H.) is now easily seen,
extending from in front of the hyoid cartilages back to the
branchial region. It is not quite straight, but curves slightly
to the side, and at the junction of the hyoid and hyomandibular
632 J. PLAYFA1E MCMCJERICH.
turns abruptly upwards towards the base of the skull, parallel
to the first branchial arch. This is probably the genio- or
basihy al.
The branchial arches (Bis.) remain the same practically as
in the last stage, and throughout show very little differentiation.
Another cartilage, however, makes its appearance at this stage,
vvhich, though not at present connected with the •visceral
cartilages, eventually unites with them. It consists (fig. 2,
E. Pa.), on either side, of a cartilaginous rod, articulating with
the sides of the rostral cartilage, near its anterior extremity,
and extends backwards and slightly downwards. It is the
commencement of the ethmopalatine cartilage.
The principal change to be noted in Stage c (fig. 5) is the
growth of the pterygo-quadrate cartilage. It has now grown
upwards and expanded at the extremity with an anterior and
posterior process. The anterior is clearly connected with the
ethmo-palatiue by a band of connective tissue, and represents
the pterygoid portion, while the posterior one has no connec-
tions, but probably is the future metapterygoid. Another
point is clearly noticeable at this stage, which helps in no
small degree to indicate the homologies of the cartilages. The
symplectic does not meet the basal portion of Meckel's carti-
lage ; the only cartilage articulating with this being the
quadrate portion of the pterygo-quadrate. In previous stages
this meeting and articulation seems to exist, or, at all events,
the two cartilages are almost in contact, but now their want of
union can be clearly seen.
The growth of the pterygo-quadrate constitutes again the
most noticeable feature when we examine Stage D. In this it
is seen that the pterygoid process has grown so far forward,
and the ethmopalatine so far backward, that they are now
separated only by a very small portion of connective tissue.
The growth has been mainly, however, on the part of the
pterygoid process. The metapterygoid process has grown back-
wards only a short distance, and at this stage does not form a
buttress to the hyomandibular, as in the salmon. In the pre-
ceding stage, but more clearly to be seen in this, one can