GENERAL ACCOUNT OF THE SKELETON IN AMPHIBIA.
EXOSKELETON.
The exoskeleton, at any rate in most living forms, is very slightly
developed in Amphibia. The only representatives of the epidermal
exoskeleton are (1) the minute horny beaks found coating the
premaxillae and dentaries in _Siren_ and the tadpoles of most Anura,
(2) the nails borne by the first three digits of the pes in _Xenopus_
and by the Japanese Salamander _Onychodactylus_, (3) the horny
covering of the calcar or prehallux of frogs. The Urodela and nearly
all the Anura, which form the vast majority of living Amphibia, have
naked skins. A few Anura belonging to the genera _Ceratophrys_ and
_Brachycephalus_ have bony dermal plates developed in the skin of the
back, and these plates become united with some of the underlying
vertebrae.
In the Gymnophiona the integument bears small cycloid scales arranged
in rings which are equal in number to the vertebrae. These scales
contain calcareous concretions. Scales also occur between the
successive rings.
In the Labyrinthodontia the dermal exoskeleton is in many genera
greatly developed. It is generally limited to the ventral surface and
consists principally of a buckler formed of three bony plates, one
median and two lateral. These plates protect the anterior part of the
thorax, and are closely connected with the adjacent endoskeleton. They
probably represent the interclavicle and clavicles. Behind this
buckler numerous scutes are generally developed, which often cover the
whole ventral surface, and may cover the whole body.
TEETH[59].
In Amphibia teeth are generally present on the maxillae, premaxillae and
vomers, and except in Anura on the dentaries; sometimes they occur on
the palatines as in many Urodela, most Labyrinthodontia, and the
Gymnophiona; less commonly on the pterygoids as in _Menobranchus_,
_Siredon_, some Labyrinthodontia, and _Pelobates cultripes_[60], or on
the splenials as in _Siren_ and _Menobranchus_, or parasphenoid as in
_Pelobates cultripes_, _Spelerpes belli_ and _Batrachoseps_. In some
Anura such as _Bufo_ and _Pipa_ the jaws are toothless.
In Gymnophiona, _Menobranchus_, and _Siredon_, the teeth are arranged
in two concentric curved rows. The teeth of the outer row are borne on
the premaxillae and maxillae if present, (the maxillae are absent in
_Menobranchus_), the teeth of the second row on the vomers and
pterygoids in _Menobranchus_ and _Siredon_, and on the vomers and
palatines in Gymnophiona. In some Gymnophiona there is a double row of
mandibular teeth. The vomerine, palatine and parasphenoid teeth of all
forms are numerous and are not arranged in rows.
The teeth of all living Amphibia are simple conical structures
ankylosed to the bone, and consisting of dentine, coated or capped
with a thin layer of enamel. In the Labyrinthodontia teeth of more
than one size are sometimes present. The dentine of the basal part of
the larger teeth is in some genera very greatly folded, causing the
structure to be highly complicated. These folds, the intervals between
which are filled with cement, radiate inwards from the exterior and
outwards from the large pulp cavity. The basal part of the teeth of
_Ceratophrys_ (Anura) has a similar structure.
ENDOSKELETON.
VERTEBRAL COLUMN.
Four regions of the vertebral column can generally be recognised in
Amphibia, viz. the cervical, the trunk or thoraco-lumbar, the sacral
and the caudal regions. In the limbless Gymnophiona, however, only
three regions, the cervical, thoracic, and post-thoracic can be made
out. The cervical region is limited to a single vertebra which
generally differs from the others in having no transverse processes or
indication of ribs. It is generally called the atlas, but it commonly
bears a small process arising from the anterior face of the centrum
which resembles the odontoid process of higher animals, and renders it
probable that the first vertebra of Amphibia corresponds to the axis,
not to the atlas. Amphibia generally have a single sacral vertebra.
Three elements go to make up the vertebral column in Amphibia, viz.
1. the notochord,
2. the long vertebral centra,
3. intervertebral cartilage which forms the joints between successive
centra.
The relations which these three elements bear to one another are
subject to much variation. The successive stages can be well traced in
the Urodela.
1. The first stage is found in larval Urodeles in general and in adult
Ichthyoidea, and some Salamandrina. In these forms the notochord
persists and retains approximately the same diameter throughout the
whole length of the vertebral column. Bony biconcave centra are
present and constrict it to a certain extent vertebrally, while
intervertebrally there is a development of cartilage. The connection
between the bony vertebrae is effected mainly by the expanded
notochord.
2. In the next stage, as seen in _Gyrinophilus porphyriticus_, the
growth of intervertebral cartilage has caused the almost complete
obliteration of the notochord intervertebrally, and its entire
disappearance vertebrally, i.e. in the centre of each vertebra. The
intervertebral cartilage now forms the main connection between
successive vertebrae, and sometimes cases are found whose condition
approaches that of definite articulations. Readily recognisable
remains of the notochord are still found at each end of the
intervertebral constriction.
3. In the third stage differentiation and absorption of the
intervertebral cartilage has given rise to definitely articulating
opisthocoelous vertebrae. These are found in most adult Salamandrina.
* * * * *
The transverse processes of the earlier trunk vertebrae are divided
into two parts, a dorsal part which meets the tubercular process of
the rib and is derived from the neural arch, and a ventral part which
meets the capitular process of the rib, and is derived from the
ventral or haemal arch. In the caudal vertebrae and often also in the
posterior trunk vertebrae the two processes are fused.
_Siren_ and _Proteus_, although they possess minute posterior limbs,
have no sacral vertebrae, while _Cryptobranchus lateralis_ has two.
The caudal vertebrae, except the first, have haemal arches very
similar to the neural arches.
In Labyrinthodontia the centra of the vertebrae are generally well
ossified biconcave discs. In some forms however, like _Euchirosaurus_,
the centra are imperfectly ossified, and consist of bony rings
traversed by a wide notochordal canal. Each ring is formed of four
pieces, a large well-ossified neural arch, a basal piece, and a pair
of lateral pieces. Vertebrae of this type are called _rachitomous_.
In the tail region of other forms each vertebra consists of an
anterior centrum bearing the neural arch, and a posterior
intercentrum[61] bearing chevron bones. Vertebrae of this type are
called _embolomerous_. Haemal arches similar to the neural arches are
often found as in Urodela. The transverse processes are sometimes well
developed and are divided into tubercular and capitular portions.
In Gymnophiona the vertebrae are biconcave and are very numerous, they
sometimes number about two hundred and thirty. Only quite the last few
are ribless and so can be regarded as post-thoracic vertebrae. The
first vertebra has nothing of the nature of an odontoid process.
In Anura the number of vertebrae is very greatly reduced, only nine
and the urostyle being present. Of these, eight are presacral and one
sacral. The urostyle is post-sacral and corresponds to three or more
modified vertebrae. The first vertebra is without transverse
processes, the remaining presacral vertebrae have the transverse
processes fairly large, while the sacral vertebra has them very large,
forming in some genera widely expanded plates. The urostyle is a long
cylindrical rod which articulates with the sacrum generally by two
facets. Ankylosed to its anterior end are the remains of two neural
arches.
In Anura remains of the notochord are found in the centre of each
vertebra, i.e. vertebrally, while in the Urodela they only occur
intervertebrally.
The vertebrae in Anura are, as a rule, procoelous. The eighth vertebra
is however generally amphicoelous, while the ninth commonly has one
convexity in front, and two behind.
In some forms such as _Bombinator_, _Pipa_, _Discoglossus_ and
_Alytes_ they are opisthocoelous; in others like _Pelobates_ they are
variable.
THE SKULL[62].
CRANIUM AND MANDIBLE.
In the Amphibian skull there are as a rule far fewer bones than in the
skull of bony fish. The primordial cartilaginous cranium often
persists to a great extent. Only quite a few ossifications take place
in it; namely in the occipital region--the exoccipitals, further
forwards--the pro-otics, and at the boundary of the orbital and
ethmoidal regions--the sphenethmoid. The basi-occipital and
basisphenoid are never ossified. As in Mammalia there are two
occipital condyles formed by the exoccipitals.
Large vacuities commonly occur in the cartilage of both floor and roof
of the primordial cranium. These are roofed over to a greater or less
extent by the development of membrane bone. Thus on the roof of the
cranium there are paired parietals, frontals, and nasals, and on its
floor are paired vomers, and a median unpaired parasphenoid.
In all living forms the parietals meet and there is no interparietal
foramen, though this exists in Labyrinthodonts.
The palato-pterygo-quadrate bar is united at each end with the
cranium, but elsewhere in most cases forms a wide arch standing away
from it. The suspensorium is, as in Dipnoi and Holocephali,
autostylic. The palato-pterygo-quadrate bar sometimes remains entirely
cartilaginous, sometimes its posterior half is ossified forming the
quadrate. In connection with it a number of membrane bones are
generally developed, viz. the maxillae, premaxillae, palatines,
pterygoids, quadratojugals, and squamosals. The pterygoids are,
however, sometimes partially formed by the ossification of cartilage.
The cartilage of the lower jaw and its investing membrane bones
generally have much the same relations as in bony fishes.
URODELA. The skulls of the various Urodeles show an interesting series
of modifications and differ much from one another, but all agree in
the absence of the quadratojugals, in the fact that the palatines lie
parallel to the axis of the cranium, and in the large size of the
parasphenoid.
The lower types _Menobranchus_, _Siren_, _Proteus_, and _Amphiuma_
have longer and narrower skulls than do the higher types.
_Menobranchus_ has a very low type of skull which remains throughout
life in much the same condition as that of a young tadpole or larval
salamander. The roof and floor of the cranium internal to the membrane
bones are formed of fibrous tissue, not of well-developed cartilage.
The epi-otic regions of the skull are ossified, forming a pair of
large bones which lie external to, and distinct from, the
exoccipitals. _Proteus_ and the Labyrinthodonts are the only other
Amphibia which have these elements separately ossified. The parietals
send a pair of long processes forwards along the sides of the
frontals. Nasals and maxillae are absent, as is likewise the case in
_Proteus_. Teeth are borne on the vomers, premaxillae, pterygoids,
dentaries and angulo-splenials. The suspensorium is forwardly
directed.
The skull of _Siren_ resembles that of _Menobranchus_ in several
respects, as in the forward direction of the suspensorium and in the
absence of maxillae, but differs in the possession of nasals, in the
toothless condition of the premaxillae and dentaries, and in the fusion
and dentigerous condition of the vomers and palatines.
_Amphiuma_ has a skull which, though narrow and elongated, differs
from those of _Menobranchus_, _Proteus_, and _Siren_, and resembles
those of higher types in the following respects:--
(1) the suspensorium projects nearly at right angles to the cranium
instead of being directed forwards, (2) the maxillae are well
developed, and the premaxillae are completely ankylosed together, (3)
there are no palatines.
The skulls of _Megalobatrachus_, _Cryptobranchus_ and _Siredon_
resemble those of the highest Urodeles the Salamanders in their wide
form, in having the pro-otics distinct from the exoccipitals which are
ossified continuously with the epi-otics and opisthotics, and in
having no palatines, but differ in having the two premaxillae
separate, and in the arrangement of the vomerine teeth which in
_Megalobatrachus_ and _Cryptobranchus_ are placed along the anterior
boundaries of the bones, these meeting in the middle line. In
_Siredon_ the vomers are separated by the very large parasphenoid.
The suspensorium in _Megalobatrachus_ and _Cryptobranchus_ projects at
right angles to the cranium; in _Siredon_ it projects somewhat
downwards and forwards as in the Salamandrina.
Modifications of the vomers, pterygoids and palatines accompany the
changes of the larval ichthyoid _Siredon_ into the adult salamandroid
_Amblystoma_, the vomers especially come to resemble to a much greater
extent those of the Salamandrina.
The ossification of the skull in the Salamandrina is carried further
than in the Ichthyoidea, though the supra-occipital and basi-occipital
are not ossified. The skull differs from that in the Ichthyoidea in
the size of the part of the vomero-palatines which lies in front of
the teeth, in the frequent union of the two premaxillae and in the
ossification of all the periotic bones continuously with the
exoccipital.
The skull differs from that of Anura in the following respects:--
(1) the bones of the upper jaw do not form a complete arch standing
away from the cranium, and the maxillae are not united to the quadrates
by quadratojugals, (2) the long axis of the suspensorium passes
obliquely downwards and forwards instead of downwards and backwards,
(3) there is no sphenethmoid encircling the anterior end of the
brain, its place being partly taken by a pair of orbitosphenoids, (4)
there is no definite tympanic cavity.
[Illustration FIG. 27. DORSAL VIEW OF THE SKULL OF A LABYRINTHODONT
(_Capitosaurus nasutus_) × 1/9 (from VON ZITTEL).
1. premaxillae.
2. nasal.
3. maxillae.
4. anterior nares.
5. frontal.
6. prefrontal.
7. lachrymal.
8. jugal.
9. orbit.
10. parietal.
11. postfrontal.
12. postorbital.
13. interparietal foramen.
14. squamosal.
15. supratemporal.
16. quadratojugal.
17. quadrate.
18. epi-otic.
19. dermo-supra-occipital.
20. exoccipital.
21. foramen magnum.]
LABYRINTHODONTIA. The skull in Labyrinthodontia is remarkable for its
extreme solidity, the large number of bones which are present, and the
extent to which the roofing over of the temporal and other fossae has
taken place. In many forms the surface of the bones is as in
Crocodiles, strongly sculptured (fig. 27, right half) with ridges and
grooves which probably lodged sensory organs. The bones forming the
roof of the skull are generally very uniform in size, perhaps the most
noticeable of them being the paired dermo-supra-occipitals (fig. 27,
19). Paired dermo-supra-occipitals occur also in certain Ganoids. The
Labyrinthodont skull also bears resemblance to that of many fish in
the development of a pair of long pointed epi-otics (fig. 27, 18),
which remain permanently distinct from the surrounding bones. The
parietals are small and enclose between them the interparietal foramen
(fig. 27, 13). In some forms in which the head is protected with an
armour of scutes, these do not roof over the interparietal foramen,
and from this fact it has been inferred that the Labyrinthodonts had a
functional pineal eye. Both supra- and infra-temporal fossae are
partially or completely roofed over by the postorbitals and large
supra-temporals (fig. 27, 15).
There is generally a ring of bones in the sclerotic coat of the eye.
The pterygoids do not meet in the middle line, being separated by the
parasphenoid. The palatines bear teeth, and in some genera
(_Archegosaurus_) form long splints lying along the inner side of the
maxillae and more or less surrounding the posterior nares. In others
(_Nyrania_) they lie in the normal position near the middle line, one
on each side of the parasphenoid. The vomers bear teeth and sometimes
meet in the middle line; they are sometimes confluent with the
parasphenoid. On the ventral surface of the cranium there are
generally large palatal vacuities.
In the mandible there is often a well-marked postglenoid process, and
the articular is generally completely ossified.
[Illustration FIG. 28. A, VENTRAL VIEW OF THE CRANIUM; B, LATERAL VIEW
OF THE CRANIUM AND MANDIBLE OF _Siphonops annulatus_ (after
WIEDERSHEIM).
1. anterior nares.
2. naso-premaxillae.
3. frontal.
4. parietal.
5. maxillae.
6. vomer.
7. orbit.
8. quadrate united with the pterygoid in front.
9. squamosal.
10. exoccipital.
11. dentary.
12. angular.
13. basi-occipital and basisphenoid fused.
14. posterior narial opening surrounded by the palatine.
X. pneumogastric foramen.]
GYMNOPHIONA. The skull bears a considerable resemblance to that of
Labyrinthodonts, especially in the arrangement of the bones which
bound the mouth cavity. The cranium is very hard, and is covered by
a complete bony roof formed mainly of the exoccipitals, parietals,
frontals, prefrontals, nasals and premaxillae. The nasals and
premaxillae are sometimes ossified continuously. There is a median
unpaired ethmoid whose dorsal end appears at the surface wedged in
between the frontals and parietals. The bone generally regarded as the
squamosal[63] is very large, and it and the maxillae generally together
surround the orbit, which, in _Epicrium_, has in it a ring of bones.
The palatines form long tooth-bearing bones fused with the inner sides
of the maxillae; they nearly surround the posterior nares.
The quadrate bears the knob, and the angular the cup for the
articulation of the mandible,--a very primitive feature. The mandible
is also noticeable for the enormous backward projection of the
angular.
* * * * *
ANURA. In Anura the skull is very short and wide owing to the
transverse position of the suspensorium. There is often a small
ossification representing the quadrate. Sometimes as in _Hyla_ and
_Alytes_ there is a fronto-parietal fontanelle.
As compared with the skull in Urodela the chief characteristics of the
skull of Anura are:--
1. the presence of a sphenethmoid,
2. the union of the frontals and parietals on each side,
3. the occasional occurrence of small supra- and basi-occipitals,
4. the backward growth of the maxillae and its connection with the
suspensorium by means of the quadratojugal,
5. the dagger-like shape of the parasphenoid,
6. the occurrence of a definite tympanic cavity,
7. the frequent occurrence of a predentary or mento-meckelian
ossification in the mandible.
The skull of _Pipa_ is abnormal, being greatly flattened and
containing little cartilage. The fronto-parietals are fused, and there
is no sphenethmoid. The quadrates are well developed and the
squamosals and parasphenoid differ much from those of other Anura.
HYOID AND BRANCHIAL ARCHES.
In larval Amphibia the hyoid and four branchial arches are generally
present, and in adult Ichthyoidea they are frequently almost as well
represented as in the larva, and are of use in strengthening the
swallowing apparatus. They are very well seen in _Siredon_, and
consist of a hyoid attached by ligaments to the suspensorium, followed
by four branchial arches of which the first and second are united by a
copula (fig. 29, D, 8), while the third and fourth are not. The hyoid
is not always the largest and best preserved of the arches, for
sometimes as in _Spelerpes_ one of the branchials is far larger than
the hyoid. Four branchial arches occur in _Siren_ as in _Siredon_, but
in _Proteus_ there are only three.
In some larval Labyrinthodontia (_Branchiosaurus_) four branchial
arches are known to occur, and their arrangement is almost precisely
similar to that in _Siredon_.
In Gymnophiona the remains of only three branchial arches occur in
addition to the hyoid. The four arches are all very similar to one
another, each consists of a curved rod of uniform diameter throughout.
The hyoid is united with the first branchial arch, but has no
attachment to the cranium.
In larval Anura (fig. 29, C) the arrangement of the hyoid and
branchial arches is much as in Urodela. In the adult, however, the
ventral parts of all the arches unite, forming a compact structure,
the _basilingual plate_ (fig. 29, B, 1).
[Illustration FIG. 29. VISCERAL ARCHES OF AMPHIBIA.
A. _Molge cristata_ (after PARKER).
B. _Rana temporaria_ adult (after PARKER).
C. Tadpole of _Rana_ (after MARTIN ST ANGE).
D. _Siredon pisciformis_ (after CREDNER).
In each case the ossified portions are slightly shaded, while the
cartilaginous portions are left white.
1. basilingual plate.
2. hyoid arch.
3. first branchial arch.
4. second do.
5. third branchial arch.
6. fourth do.
7. thyro-hyal.
8. copula.]
The dorsal parts of the first three branchial arches disappear, but
those of the fourth become ossified and form the short, stout
thyro-hyals or posterior cornua. The dorsal parts of the hyoid arch in
the adult form a pair of long bars, the anterior cornua, which are
united to the periotic region of the skull in front of the fenestra
ovalis either by short ligaments or by fusion as in _Bufo_. In _Pipa_
and _Xenopus_ the first and second branchial arches persist as well as
the fourth (thyro-hyal), but in _Pipa_ the hyoid is wanting.
RIBS.
Ribs are generally very poorly developed in Amphibia. In Anura they
are in most cases absent; when present they generally form minute
unossified appendages attached to the transverse processes, but in
_Discoglossus_ and _Xenopus_ the anterior vertebrae are provided with
distinct ribs. In Urodela and Labyrinthodontia they are generally
short structures, each as a rule attached to the vertebra by a
bifurcated proximal end. The number of rib-bearing vertebrae varies,
but the first and the posterior caudal vertebrae are always ribless.
The anterior caudal vertebrae too are generally ribless, but sometimes
a few of them bear small ribs. In _Spelerpes_ the last two trunk
vertebrae are ribless, and hence may be regarded as lumbar vertebrae.
In Gymnophiona ribs are better developed than in any other Amphibia;
they occur on all the vertebrae except the first and last few, and are
attached to the transverse processes, sometimes by single, sometimes
by double heads.
Sternal ribs are almost unknown in Amphibia, but traces of them occur
in _Menobranchus._
STERNUM.
In Amphibia the sternum is not very well developed; sometimes as in
Gymnophiona and _Proteus_ no traces of it occur, and in the Urodela it
is never ossified. It is always very intimately related to the
pectoral girdle. In the Salamandrina it has the form of a broad thin
plate of cartilage, grooved and overlapped by the coracoid.
[Illustration FIG. 30. SHOULDER-GIRDLE AND STERNUM OF
A. An old male common Frog (_Rana temporaria_).
B. An adult female _Docidophryne gigantea_ (after PARKER).
In both A and B the left suprascapula is removed. The parts left
unshaded are ossified; those marked with small dots consist of hyaline
cartilage, those marked with large dots of calcified cartilage.
1. calcified cartilage of suprascapula.
2. ossified portion of suprascapula.
3. scapula.
4. coracoid.
5. epicoracoid.
6. precoracoid.
7. clavicle.
8. glenoid cavity.
9. coracoid foramen.
10. episternum.
11. omosternum.
12. sternum.
13. xiphisternum.]
In most Anura the sternum consists of a number of parts arranged in
series. At the anterior end is a flat cartilaginous plate with a
bony basal stalk. This plate is called the episternum, and its stalk
the omosternum. The continuity of the sternum is now interrupted
by a pair of cartilaginous structures, the epicoracoids, which are
shoulder-girdle elements, and represent the unossified ventral ends of
the coracoids. In some cases cartilaginous epiprecoracoids can also
be distinguished. Further back is the long sternum proper, while last
comes the xiphisternum, a broad expanded plate of cartilage.
In some Anura such as _Pipa_ and _Hyla_ the number of sternal elements
is considerably reduced.
APPENDICULAR SKELETON.
PECTORAL GIRDLE.
The most primitive Amphibian shoulder-girdle is found in the Urodela.
It consists of a dorsal element, the scapula, a posterior ventral
element, the coracoid, and an anterior ventral element, the
precoracoid. The clavicle is not developed, and the two coracoids
overlap in the middle line. The shoulder-girdle remains largely
cartilaginous but the proximal end of the scapula is ossified, and the
ossification may extend through part of the coracoid and precoracoid.
In Labyrinthodontia there is an exoskeletal ventral buckler formed of
three plates, a median one, which probably represents an
interclavicle, and two lateral ones, which are probably clavicles.
Traces of endoskeletal structures, probably corresponding to the
precoracoid and scapula, are also known in some cases. The Gymnophiona
and some of the Labyrinthodontia have lost the pectoral girdle and
limbs.
The ossification of the shoulder-girdle has gone on much further in
Anura than it has in Urodela. Clavicles are present and the scapula
and coracoid of each side are ossified from separate centres. The
distal part of the scapula forms a large imperfectly ossified plate,
the suprascapula.
The shoulder-girdle of Anura is however subject to considerable
variations. In the Toads (Bufonidae) the epicoracoids or unossified
ventral ends of the coracoids and precoracoids overlap in the middle
line (fig. 30, B, 5). This arrangement is called _Arciferous_. In the
Frogs,--Ranidae, and other forms belonging to the group
_Firmisternia_,--the epicoracoids do not overlap but form a narrow
cartilaginous bar separating the ventral ends of the coracoids (fig.
30, A, 5).
ANTERIOR LIMB.
In many Amphibia and especially in the Urodela the anterior limb has a
very simple unmodified arrangement. The humerus is straight and of
moderate length, its ends are rounded for articulation on the one hand
with the shoulder-girdle, and on the other hand with the radius and
ulna. In the Urodela the radius and ulna are distinct. In the Anura
they have fused, though the line of junction of the two is not
obliterated. Their proximal ends are hollowed for articulation with
the convex end of the humerus.
The manus in all recent Amphibia agrees in never having more than four
complete digits, but is subject to considerable variation, this
statement applying especially to the carpus.
In the larva of Salamandra (fig. 31, A), except that the pollex is
absent[64], the manus retains completely the condition which is
generally regarded as primitive for the higher Vertebrata. It consists
of an anterior row of three elements, the ulnare, intermedium, and
radiale, and a posterior row of four, the carpalia 2, 3, 4, and 5.
Interposed between the two rows is a centrale. _Menobranchus_ has a
similar very simple carpus. In most other Amphibia this simplicity is
lost. This loss may be due to:--
(_a_) fusion of certain structures, e.g. in the adult _Salamandra_ the
intermedium and ulnare have fused,
(_b_) displacement of structures, e.g. in _Bufo viridis_, the centrale
has been pushed up till it comes to articulate with the radius,
(_c_) the development of supernumerary elements, especially of extra
centralia. In _Megalobatrachus_ two or even three centralia sometimes
occur.
[Illustration FIG. 31. A, RIGHT ANTIBRACHIUM AND MANUS OF A LARVAL
SALAMANDER (_Salamandra maculosa_) (after GEGENBAUR).
B, RIGHT TARSUS AND ADJOINING BONES OF _Molge sp._ (after GEGENBAUR).
1. radius.
2. ulna.
3. radiale.
4. intermedium.
5. ulnare.
6. centrale.
7. carpale 2.
8. " 3.
9. " 4.
10. " 5.
11. tibia.
12. fibula.
13. tibiale.
14. intermedium.
15. fibulare.
16. centrale.
17. tarsale 1.
18. tarsalia 4 and 5 fused.
I. II. III. IV. V. digits.]
In the great majority of Amphibia while one digit, probably the first,
is absent, the other four digits are well developed. In the forms
however with degenerate limbs like _Amphiuma_, _Siren_ and _Proteus_
the number of digits is still further reduced. In _Siren_ there are
three or four, in _Proteus_ three, and in _Amphiuma_ two or three
digits in the manus.
In Anura the pollex is represented only by a short metacarpal. There
are sometimes traces of a prepollex. The carpus often has two
centralia and the intermedium is absent.
In Labyrinthodontia the limbs are generally very simple and resemble
those of Urodela. In some forms, however, the manus differs from that
of all living Amphibia in possessing five well-developed digits.
PELVIC GIRDLE.
The simplest Amphibian pelvis is that of some of the Labyrinthodontia;
thus in _Mastodonsaurus_ it consists dorsally of a short broad ilium
placed vertically and attached to the sacrum, and ventrally of a small
pubis and of a large ischium meeting its fellow in the middle line. In
some Labyrinthodonts the pubes as well as the ischia meet in a ventral
symphysis, and in many there are no obturator foramina. In _Siren_,
Gymnophiona and some Labyrinthodontia the pelvic girdle and limbs are
absent.
In Urodela the ventral element of the pelvis on each side forms a flat
plate which meets its fellow of the opposite side. The anterior part
of the plate, representing the pubis, generally remains cartilaginous
throughout life; the posterior part representing the ischium is in
almost every case well ossified. Attached to the anterior end of the
pubes there is an unpaired bifid cartilaginous structure, the
epipubis. The ilia are vertically placed.
In most Anura the pelvis is peculiarly modified in correlation with
the habits of jumping. The long bone generally called the ilium is
placed horizontally and is attached at its extreme anterior end to the
sacrum. The ischium is ossified and distinct. Ventrally in front of
the ischium there is a tract of unossified cartilage which is often
regarded as the pubis. In _Xenopus_, however, the bone corresponding
to the ilium of the Frog is seen to ossify from two centres, one
forming the ilium, the other, which lies at the symphysis, being
apparently the pubis. This makes it probable that the so-called ilium
of the Frog is really to be regarded as an ilio-pubis, and renders the
homology of the cartilaginous part uncertain, but it probably
corresponds to the acetabular bone of mammals. In _Xenopus_ also there
is a minute epipubis similar to that of Urodeles.
POSTERIOR LIMB.
In Urodela the posterior limb (fig. 31, B) closely resembles the
anterior limb, but is even less removed from the primitive condition
of the higher vertebrates in the fact that all five digits are
commonly present. The tibia and fibula are short bones approximately
equal in size. In some cases the number of digits is reduced. Thus in
_Menobranchus_ the pes has four digits, in _Proteus_ it has two, and
in _Amphiuma_ two or three, while in _Siren_ the posterior limbs have
atrophied.
In correlation with their habits of jumping, the posterior limbs in
Anura are much lengthened and considerably modified. The tibia and
fibula are completely fused. The intermedium is absent, while the
tibiale and fibulare are greatly elongated. Tarsalia 4 and 5 are
absent. Five digits are always present, and there is a prehallux
formed of two or more segments.
In general the posterior limbs in Labyrinthodontia bear the closest
resemblance to the anterior limbs; in some cases three centralia are
found.
In Ichthyoidea, and in most Labyrinthodontia, the cartilages of the
carpus and tarsus remain unossified; in Salamandrina and in Anura they
are generally ossified.
FOOTNOTES:
[59] O. Hertwig. Ueber das Zahnsystem der Amphibien. _Arch. mikr.
Anat._ supplem. Bd. XI. 1875.
[60] G.A. Boulenger, _P.Z.S._ 1890, p. 664.
[61] See p. 14.
[62] See many papers by W.K. Parker published in the _Phil. Trans._ of
the Royal Soc.
[63] Perhaps this bone includes supra-orbital and postorbital
elements.
[64] The first digit present is sometimes regarded as the pollex, but
from analogy with Anura it is probable that the pollex is the missing
digit.