SAURIA (_CONTINUED_)—OPHIDIA—SNAKES 581
INDEX 651
{xi}SCHEME OF THE CLASSIFICATION ADOPTED IN THIS BOOK
CLASS AMPHIBIA.
Sub-Class. Order. Sub-Order. Family. Sub-Family.
STEGOCEPHALI (p. 78)
STEGOCEPHALI LEPOSPONDYLI (p. 80)
BRANCHIOSAURI (p. 80).
AISTOPODES (p. 81).
STEGOCEPHALI TEMNOSPONDYLI (p. 81)
STEGOCEPHALI TEREOSPONDYLI (p. 83)
LISSAMPHIBIA (p. 84)
APODA (p. 84)
COECILIIDAE (p.89).
URODELA (p. 94)
AMPHIUMIDAE (p. 97).
SALAMANDRIDAE (p. 102)
Desmognathinae (p. 102).
Plethodontinae (p. 103).
Amblystomatinae (p. 109).
Salamandrinae (p. 115).
PROTEIDAE (p. 132).
SIRENIDAE (p. 136).
ANURA (p. 138)
AGLOSSA (p. 143).
PHANEROGLOSSA (p. 152)
DISCOGLOSSIDAE (p. 152).
PELOBATIDAE (p. 160).
BUFONIDAE (p. 166).
HYLIDAE (p. 185)
Amphignathodontinae
(p. 188).
Hylinae (p. 189).
CYSTIGNATHIDAE (p. 209)
Hemiphractinae (p. 210).
Cystignathinae (p. 211).
Dendrophryniscinae
(p. 224).
ENGYSTOMATIDAE (p. 225)
Engystomatinae (p. 225).
Dyscophinae (p. 235).
Genyophryninae (p. 236).
RANIDAE (p. 237)
Ceratobatrachinae
(p. 237).
Raninae (p. 238).
Dendrobatinae (p. 272).
CLASS REPTILIA (p. 277).
PROREPTILIA (p. 285). _Eryops_ (p. 286). _Cricotus_ (p. 287).
Sub-Class. Order. Sub-Order. Family. Sub-Family.
PROSAURIA (p. 288)
MICROSAURI (p. 288).
PROSAURI (p. 290).
PROTOROSAURI (p. 290).
RHYNCHOCEPHALI (p. 292).
THEROMORPHA (p. 300)
PAREIASAURI (p. 304).
THERIODONTIA (p. 306).
ANOMODONTIA (p. 309).
PLACODONTIA (p. 311).
CHELONIA (p. 312)
ATHECA (p. 333)
SPHARGIDAE (p. 333).
THECOPHORA (p. 338)
CRYPTODIRA (p. 338)
CHELYDRIDAE (p. 338).
DERMATEMYDIDAE (p. 341).
CINOSTERNIDAE (p. 342).
PLATYSTERNIDAE (p. 345).
TESTUDINIDAE (p. 345).
CHELONIDAE (p. 378).
PLEURODIRA (p. 388)
PELOMEDUSIDAE (p. 390).
CHELYDIDAE (p. 399).
CARETTOCHELYDIDAE (p. 404).
TRIONYCHOIDEA (p. 404)
TRIONYCHIDAE (p. 404).
DINOSAURIA (p. 412)
SAUROPODA (p. 418).
THEROPODA (p. 420).
ORTHOPODA (p. 424)
STEGOSAURI (p. 425).
ORNITHOPODA (p. 426)
CERATOPSIA (p. 430).
CROCODILIA (p. 431)
PSEUDOSUCHIA (p. 432).
PARASUCHIA (p. 433).
EUSUCHIA (p. 434)
TELEOSAURIDAE (p. 450).
METRIORHYNCHIDAE (p. 451).
MACRORHYNCHIDAE (p. 451).
GAVIALIDAE (p. 451).
ATOPOSAURIDAE (p. 453).
GONIOPHOLIDAE (p. 453).
CROCODILIDAE (p. 454).
PLESIOSAURIA (p. 473)
NOTHOSAURI (p. 476)
MESOSAURIDAE (p. 476).
NOTHOSAURIDAE (p. 477).
PLESIOSAURI (p. 477)
PLIOSAURIDAE (p. 477).
PLESIOSAURIDAE (p. 478).
ELASMOSAURIDAE (p. 478).
ICHTHYOSAURIA (p. 478)
ICHTHYOSAURI (p. 483).
PTEROSAURIA (p. 484)
PTEROSAURI (p. 486)
PTERODACTYLI (p. 486).
PTERANODONTES (p. 487).
PYTHONOMORPHA (p. 487)
DOLICHOSAURI (p. 489).
MOSASAURI (p. 489).
SAURIA (p. 491)
LACERTILIA (p. 491)
GECKONES (p. 502)
GECKONIDAE (p. 507)
Geckoninae (p. 507).
Eublepharinae (p. 512).
Uroplatinae (p. 512).
LACERTAE (p. 513)
AGAMIDAE (p. 515).
IGUANIDAE (p. 528).
XENOSAURIDAE (p. 536).
ZONURIDAE (p. 536).
ANGUIDAE (p. 537).
HELODERMATIDAE (p. 540).
LANTHANOTIDAE (p. 541).
VARANIDAE (p. 542).
XANTUSIIDAE (p. 547).
TEJIDAE (p. 547).
LACERTIDAE (p. 549).
GERRHOSAURIDAE (p. 559).
SCINCIDAE (p. 559).
ANELYTROPIDAE (p. 564).
DIBAMIDAE (p. 564).
ANIELLIDAE (p. 564).
AMPHISBAENIDAE (p. 565).
PYGOPODIDAE (p. 567).
CHAMAELEONTES (p. 567)
CHAMAELEONTIDAE (p. 573).
OPHIDIA (p. 581)
TYPHLOPIDAE (p. 593).
GLAUCONIIDAE (p. 594).
ILYSIIDAE (p. 594).
UROPELTIDAE (p. 595).
BOIDAE (p. 596)
Pythoninae (p. 598).
Boinae (p. 601).
XENOPELTIDAE (p. 605).
COLUBRIDAE (p. 606)
Aglypha (p. 606)
Acrochordinae (p. 606).
Colubrinae (p. 607).
Rhachiodontinae (p. 622).
COLUBRIDAE
Opisthoglypha (p. 623)
Dipsadomorphinae
(p. 623).
Elachistodontinae
(p. 625).
Homalopsinae (p. 625).
COLUBRIDAE
Proteroglypha (p. 625)
Elapinae (p. 626).
Hydrophinae (p. 635).
AMBLYCEPHALIDAE (p. 637).
VIPERIDAE (p. 637)
Viperinae (p. 638).
Crotalinae (p. 644).
PART I
AMPHIBIA
"'s scheint, dass die hier oben keine
Ahnung haben von dem Sumpf und
Seiner Pracht."
The "plattgedrückte Kröte,"
SCHEFFEL'S _Trompeter von Säkkingen_.
{3}CHAPTER I
AMPHIBIA
CHARACTERS AND DEFINITION–POSITION OF THE CLASS AMPHIBIA IN THE PHYLUM
VERTEBRATA–HISTORICAL ACCOUNT OF THE CLASSIFICATION OF AMPHIBIA
A bird is known by its feathers, a Beast by its hairs, a Fish by its fins,
but there is no such obvious feature which characterises the Amphibia and
the Reptiles. In fact, they are neither fish, flesh, nor fowl. This
ill-defined position is indicated by the want of vernacular names for these
two classes, a deficiency which applies not only to the English language.
All the creatures in question are backboned, creeping animals. Those which
are covered with horny scales, and which from their birth breathe by lungs
only, as Crocodiles, Tortoises, Lizards, and Snakes, are the Reptiles. The
rest, for instance, Newts or Efts, Frogs and Toads, are the Amphibia. Their
skin is mostly smooth and clammy and devoid of scales; the young are
different from the adult in so far as they breathe by gills and live in the
water, before they are transformed into entirely lung-breathing,
terrestrial creatures. But there are many exceptions. _Proteus_ and _Siren_
the mud-eel, always retain their gills; while not a few frogs undergo their
metamorphosis within the egg, and never breathe by gills. If we add the
tropical limbless, burrowing Coecilians, and last, not least, the
Labyrinthodonts and other fossil forms, the proper definition of the class
Amphibia,–in other words, the reasons for grouping them together into one
class, separated from the other backboned animals,–requires the examination
of many other characters.
{4}So far as numbers of living species are concerned, the Amphibia are the
least numerous of the Vertebrata. There are about 40 limbless, burrowing
APODA; 100 URODELA or tailed two- or four-footed newts, and about 900
ANURA, or tailless, four-footed frogs and toads; in all some 1000 different
species. Few, indeed, in comparison with the 2700 Mammals, 3500 Reptiles,
nearly 8000 Fishes, and almost 10,000 Birds. But we shall see that the
Amphibia have not only "had their day," having flourished in bygone ages
when they divided the world, so far as Vertebrata were concerned, between
themselves and the Fishes, but that they never attained a dominant
position. Intermediate between the aquatic Fishes and the gradually rising
terrestrial Reptiles they had to fight, so to speak, with a double front
during the struggle of evolution, until by now most of them have become
extinct. The rest persist literally in nooks and corners of the teeming
world, and only the Frogs and Toads, the more recent branch of the
Amphibian tree, have spread over the whole globe, exhibiting almost endless
variations of the same narrow, much specialised plan. The greatest charm of
the Anura lies in their marvellous adaptation to prevailing circumstances;
and the nursing habits of some kinds read almost like fairy-tales.
CHARACTERS OF THE AMPHIBIA.[1]
1. The vertebrae are (_a_) acentrous, (_b_) pseudocentrous, or (_c_)
notocentrous.
2. The skull articulates with the atlas by two condyles which are formed
by the lateral occipitals. For exceptions see p. 78.
3. There is an auditory columellar apparatus, fitting into the fenestra
ovalis.
4. The limbs are of the tetrapodous, pentadactyle type.
5. The red blood-corpuscles are nucleated, biconvex, and oval.
6. The heart is (_a_) divided into two atria and one ventricle, and (_b_)
it has a conus provided with valves.
7. The aortic arches are strictly symmetrical.
8. Gills are present at least during some early stages of development.
9. The kidneys are provided with persistent nephrostomes.
10. Lateral sense-organs are present at least during the larval stage.
11. The vagus is the last cranial nerve.
12. The median fins, where present, are not supported by spinal skeletal
rays.
13. Sternal ribs and a costal or true sternum are absent.
14. There is no paired or unpaired medio-ventral, copulatory apparatus.
15. Development takes place without amnion and allantois.
None of these characters is absolutely diagnostic, except 1 (_c_), and
this applies only to the Anura and most of the Stegocephali.
{5}Numbers 1 (_b_), 1 (_c_), 2, 3, 4 and 12 separate the Amphibia from
the Fishes.
Numbers 1, 6 (_b_), 7, 8, 9, 11, 13, 15 separate them from the
Reptiles, Birds, and Mammals.
Number 2 separates them from the Fishes, Reptiles, and Birds.
Number 5 separates them from the Mammals.
Number 6 (_a_) separates them from the Fishes (excl. Dipnoi), Birds and
Mammals.
We can, therefore, very easily define all the Amphibia, both recent and
extinct, by a combination of the characters enumerated above. For instance,
by the combination of numbers 2, 3 or 4 with either 7, 8, 9, 11, 13 or 15.
_Amphicondylous Anamnia_ would be an absolutely correct and all-sufficient
diagnosis, but it would be of little use in the determination of adult
specimens; and the tetrapodous character is of no avail for Apoda.
_Amphicondylous animals without an intracranial hypoglossal nerve_ is a
more practical diagnosis.
In the case of living Urodela and Anura the absence of any scales in the
skin affords a more popular character; it is unfortunately not applicable
to the Apoda, many of which possess dermal scales, although these are
hidden in the imbricating transverse rings of the epidermis; and the
frequent occurrence of typical scales of both ecto- and meso-dermal
composition in many of the Stegocephali forces us to discard the scales, or
rather their absence, as a diagnostic character of the class Amphibia. The
same applies to the mostly soft, moist, or clammy, and very glandular
nature of the skin.
THE POSITION OF THE CLASS AMPHIBIA IN THE PHYLUM VERTEBRATA.–There is no
doubt that the Amphibia have sprung from fish-like ancestors, and that they
in turn have given rise to the Reptilia. The Amphibia consequently hold a
very important intermediate position. It was perhaps not a fortunate
innovation when Huxley brigaded them with the Fishes as _Ichthyopsida_,
thereby separating them more from the _Sauropsida_ (= Reptilia and Aves),
than is justifiable,–perhaps more than he himself intended. The
connecting-link, in any case, is formed by the Stegocephali; all the recent
Orders, the Apoda, Urodela, and Anura, are far too specialised to have any
claims to the direct ancestral connections. The line leading from
Stegocephali to fossil Reptiles, notably to such Proreptilia as _Eryops_
and _Cricotus_, and even to the Lepospondylous Prosauria, is extremely
gradual, and the steps are almost imperceptible. Naturally, {6}assuming
evolution to be true, there must have lived countless creatures which were
a "rudis indigestaque moles," neither Amphibia nor Reptilia, in the present
intensified sense of the systematist. The same consideration applies
equally to the line which leads downwards to the Fishes. But the great gulf
within the Vertebrata lies between Fishes and Amphibia, between absolutely
aquatic creatures with internal gills and "fins," and terrestrial,
tetrapodous creatures, with lungs and fingers and toes. On the side of the
fishes only the Dipnoi and the Crossopterygii come into consideration.
The piscine descent of the Amphibia is still proclaimed by the following
features.–(1) The possession by the heart of a long conus arteriosus,
provided with, in many cases, numerous valves, or at least (Anura) one
series at the base, another at the beginning of the truncus where the
arterial arches branch off; (2) the strictly symmetrical arrangement of
these arches; (3) the trilocular heart is still like that of the
Lung-fishes or Dipnoi; (4) the occurrence of as many as four or even five
branchial skeletal arches in the larval stage; (5) the glottis is supported
by cartilages which themselves are derivatives of posterior visceral
arches; (6) the development of the vertebrae (Stegocephali and Urodela)
from four pairs of arcualia, and the formation of the intervertebral joints
by a split across the intervertebral ring of cartilage: this feature is
unknown in Reptilia, but it occurs also in _Lepidosteus_, most probably
also in _Polypterus_; (7) the hypoglossal still retains the character of a
post-cranial or cervical spinal nerve; (8) the presence of lateral
sense-organs; (9) the possession of external gills is of somewhat doubtful
phylogenetic value, although such gills occur amongst fishes only in Dipnoi
and Crossopterygii. It is not unlikely that in the Amphibia these organs
owe their origin to entirely larval requirements, while the suctorial mouth
of the larvae of the Anura and many fishes has certainly no ancestral
meaning, but is a case of convergent development.
The usual diagnoses of the Amphibia contain the statement that they, or
most of them, undergo a metamorphosis, or pass through a larval stage. The
same applies to various fishes; while, on the other hand, the larval (not
ancestral) stage has become permanent in the Proteidae and Sirenidae; and
lastly, we cannot well speak of larvae in the viviparous _Salamandra atra_.
{7}THE EVOLUTION OF AN ADEQUATE CLASSIFICATION OF THE AMPHIBIA has been a
long process. Even their recognition as a class, separate from, and of
equal rank with that of, the Reptilia, was by no means generally accepted
until comparatively recent times. A historical sketch of the laborious,
often painful, striving for light, in France and Germany, then in England,
and lastly in America, is not without interest.
The term _Amphibia_ was invented by Linnaeus for the third class of
animals in his famous "Systema Naturae." It comprises a very queer
assembly, which, even in the 13th edition (1767), stands as follows:–
1. REPTILES PEDATI, with the four "genera" _Testudo_, _Draco_,
_Lacerta_, and _Rana_. _Lacerta_ includes Crocodiles, Lizards, and
Newts!
2. SERPENTES APODES.
3. NANTES PINNATI. Elasmobranchs, Sturgeons, Lampreys, and various
other fishes.
Laurenti, 1768, in a dissertation entitled "Specimen medicum, exhibens
Synopsin Reptilium ...," uses Brisson's term, REPTILES, and divides them
into:–
REPTILIA SALIENTIA, these are the Anura.
GRADIENTIA, namely the Urodela and Lizards.
SERPENTIA, the Snakes and the Apoda.
Brongniart, 1800, "Essay d'une classification naturelle des Reptiles,"[2]
distinguishes:–
CHELONII, SAURII, OPHIDII, BATRACHII; the last for the Frogs, Toads,
and Newts.
Latreille, 1804, "Nouveau Dict. Hist. Nat." xxiv.,[3] accepts the four
Orders of Brongniart's "Reptiles," but clearly separates the fourth
Order, "BATRACHII," from the rest by the following, now time-honoured,
diagnosis: _Doigts des pattes n'ayant pas d'ongles; des branchies, du
moins pendant un temps; des métamorphoses_. But there is not one word
about "Amphibia" in opposition to "Reptilia."
Duméril, 1806, "Zoologie analytique" (p. 90), and "Élémens de l'histoire
naturelle," 1807, divides the "Reptiles batraciens," or "Batracii," into
ECAUDATI and CAUDATI; he also introduces the terms "ANOURES" and
"URODÈLES" as their equivalents; but since these terms appear in the
French form purists do not admit their having any claim to recognition!
Oppel, 1811, "Die Ordnungen, Familien und Gattungen der Reptilien,"
establishes the term APODA for the Coeciliae, and recognises their
affinity to the Ecaudata and Caudata by removing them from the Snakes.
De Blainville, 1816, "Prodrome d'une nouvelle distribution du règne
animal"[4]–
AMPHIBIENS SQUAMIFÈRES. [The Reptilia.]
" NUDIPELLIFÈRES s. Ichthyoides. [The Amphibia.]
{8}Merrem, 1820, "Tentamen systematis Amphibiorum."
PHOLIDOTA. [The Reptilia.]
BATRACHIA: _APODA_.
_SALIENTIA_.
{ Mutabilia [with metamorphosis, _e.g._
_GRADIENTIA_ { Newts.]
{ Amphipneusta [Perennibranchiate Urodeles.]
F. S. Leuckart, 1821, "Einiges ueber die fischartigen Amphibien."[5]
MONOPNOA. [The Reptilia.]
{ with temporary gills: Ecaudata + Caudata pt.
DIPNOA. [The Amphibia] { with permanent gills: "Proteidae," _Menopoma_
{ and _Amphiuma_.
Latreille, 1825, "Familles naturelles du règne animal." The Vertebrata
are divided into _Haematherma_ and _Haemacryma_. These terms for warm and
cold-blooded creatures were later on amended by Owen to _Haematotherma_
and _Haematocrya_. The latter are divided by Latreille as follows:–
REPTILIA. Still including the Coeciliae amongst the Snakes.
AMPHIBIA { Caducibranchiata.
{ Perennibranchiata.
PISCES.
Wagler, 1830, "Systema Amphibiorum."
TESTUDINES, CROCODILI, LACERTAE, SERPENTES, ANGUES, COECILIAE, RANAE,
ICHTHYODI.
RANAE I. _AGLOSSA._
" II. _PHANEROGLOSSA_: 1. Cauda nulla. [The Anura.]
" " 2. Cauda distincta. [The Salamandridae.]
ICHTHYODI I. _ABRANCHIALES._ _Menopoma_ [_Cryptobranchus_] and
_Amphiuma_.
" II. _BRANCHIALES._ [The Perennibranchiate Urodela.]
J. Müller, 1831, "Beiträge zur Anatomie ... der Amphibien."[6]
GYMNOPHIONA, DEROTREMATA, PROTEIDAE, SALAMANDRINA, BATRACHIA.
J. Bell, 1836, Todd's "Cyclopaedia of Anatomy and Physiology," Art.
"Amphibia."
AMPHIPNEUSTA, the Perennibranchiate Urodeles; ANOURA, URODELA;
ABRANCHIA, _Menopoma_ and _Amphiuma_; APODA.
Stannius, 1856, "Handbuch der Zootomie: Anatomie der Wirbelthiere." (2nd
ed.)
AMPHIBIA MONOPNOA. The Reptilia.
AMPHIBIA DIPNOA. 1. URODELA. _PERENNIBRANCHIATA._
_DEROTREMATA_: _Amphiuma_ and _Menopoma_.
_MYCTODERA._[7]
{9} 2. BATRACHIA. _AGLOSSA._
_PHANEROGLOSSA_: Systomata =
Engystomatidae.
Bufoninae. Without manubrium sterni.
Raninae. With manubrium.
Hyloidea. With adhesive finger-discs.
_GYMNOPHIONA._
Gegenbaur, 1859, "Grundzüge der vergleichenden Anatomie."
AMPHIBIA as a separate class, equivalent to that of the REPTILIA, are
divided into the four Orders: _PERENNIBRANCHIATA_, _SALAMANDRINA_,
_BATRACHIA_, and _GYMNOPHIONA_. In the second edition of the
"Grundzüge" (1870) they are divided into _URODELA_, _ANURA_, and
_GYMNOPHIONA_.
Huxley, 1864, "The Elements of Comparative Anatomy."
MAMMALS.
SAUROIDS, subsequently changed into SAUROPSIDA = Reptilia + Aves.
ICHTHYOIDS, " " ICHTHYOPSIDA = Amphibia + Pisces.
Haeckel, 1866, "Generelle Morphologie."
Amphibia. A. PHRACTAMPHIBIA s. Ganocephala = Labyrinthodonta + Peromela
[Apoda].
B. LISSAMPHIBIA s. Sozobranchia = Sozura [Urodela] + Anura.
Cope, 1869.[8]
STEGOCEPHALI, GYMNOPHIDIA, URODELA, PROTEIDEA, TRACHYSTOMATA, ANURA.
Huxley, 1871, "A Manual of the Anatomy of Vertebrated Animals."
Amphibia I. SAUROBATRACHIA [v.d. Hoeven's term] s. URODELA
1. Proteidea.
2. Salamandridae.
II. LABYRINTHODONTA.
III. GYMNOPHIONA.
IV. BATRACHIA s. ANURA.
Boulenger, 1882, "Catalogue of the BATRACHIA GRADIENTIA s. CAUDATA and
BATRACHIA APODA," divides the Caudata simply into: _SALAMANDRIDAE_,
_AMPHIUMIDAE_, _PROTEIDAE_, and _SIRENIDAE_.
1882, "Cat. Batrachia Salientia s. Ecaudata," see p. 140.
Cope, 1890, "Synopsis of the Families of Vertebrata."[9]
CLASS BATRACHIA.
Sub-Class I. STEGOCEPHALI.
Order 1. Ganocephali: _Trimerorhachis_, _Archegosaurus_.
2. Rhachitomi: _Eryops_.
3. Embolomeri: _Cricotus_.
4. Microsauri: _Branchiosaurus_, _Hylonomus_, etc.
{10} Sub-Class II. URODELA.
Order 1. Proteidae: _Proteus_.
2. Pseudosauria. [All the rest of the Urodela +
Coeciliidae.]
3. Trachystomata: Sirenidae.
Sub-Class III. SALIENTIA.
P. and F. Sarasin, 1890, "Zur Entwicklungsgeschichte der Ceylonesischen
Blindwühle, Ichthyophis glutinosa."[10]
Sub-Class I. ARCHAEOBATRACHI s. STEGOCEPHALI.
II. NEOBATRACHI.
Order 1. _URODELA._
_a_. Salamandroidea. [The Urodela.]
_b_. Coeciloidea = Amphiumidae + Coeciliidae.
2. _ANURA._
The classification adopted in this volume is as follows:–
CLASS AMPHIBIA.
Sub-Class I. Phractamphibia.
Order I. Stegocephali Lepospondyli.
Sub-order 1. Branchiosauri.
Sub-order 2. Aistopodes.
Order II. Stegocephali Temnospondyli.
Order III. Stegocephali Stereospondyli.
Sub-Class II. Lissamphibia.
Order I. Apoda.
Order II. Urodela.
Order III. Anura.
Sub-order 1. Aglossa.
Sub-order 2. Phaneroglossa.
{11}CHAPTER II
SKELETON OF URODELA AND ANURA–SKIN–COLOUR-CHANGING MECHANISM–
POISON-GLANDS–SPINAL NERVES–RESPIRATORY ORGANS–SUPPRESSION OF LUNGS–
URINO-GENITAL ORGANS–FECUNDATION–NURSING HABITS–DEVELOPMENT AND
METAMORPHOSIS
SKELETON OF THE URODELA
THE VERTEBRAL COLUMN.–The number of vertebrae is smallest in the
terrestrial, greatest in the entirely aquatic forms, and is exceptionally
large in the eel-shaped _Amphiuma_. In the following table the sacral
vertebra is included in those of the trunk.
Trunk. Tail.
_Siren lacertina_ 22 35 +
_Necturus maculatus_ 19 29
_Proteus anguinus_ 30 28 +
_Cryptobranchus alleghaniensis_ 20 or 21 24 +
_C. scheuchzeri_ 21
_C. japonicus_ 22 22 to 26
_Amphiuma means_ 63 35 +
_Amblystoma tigrinum_ 17 or 16 32 +
_Salamandra maculosa_ 17 27
_Triton cristatus_ 17 36
_Triton taeniatus_ 14 or 15 36 +
_Triton palmatus_ 14 23 to 25
_Salamandrina perspicillata_ 15 32 to 42
_Spelerpes fuscus_ 16 23
The vertebrae of the Urodela and those of the Apoda differ from those of
all the other Tetrapoda[11] by possessing no special centra or bodies. That
part which should correspond with the centrum is formed either by the
meeting and subsequent complete co-ossification of the two chief dorsal and
ventral pairs of arcualia {12}(tail-vertebrae), or entirely by the pair of
chief dorsal arcualia. There is consequently no neuro-central suture.
Moreover, the central region of each vertebra is strongly pinched in
laterally, widening towards the ends. Another feature of the vertebral
column of the Urodela is the possession of a considerable amount of
intervertebral cartilage, by which the successive vertebrae are held
together. This cartilage does not ossify, and it either remains continuous,
serving in its entirety and owing to its flexibility as a joint, or it
becomes more or less imperfectly separated into a cup and ball portion, the
cup belonging to the posterior end of the vertebra. Such joints are called
opisthocoelous, and occur in the Desmognathinae and Salamandrinae. In the
adult the cup and ball frequently calcify, and the chorda dorsalis or
notochord is completely destroyed. Those vertebrae between which the
intervertebral cartilage remains unbroken, are called amphicoelous, since
in them, most obviously in macerated or dried skeletons, the vertebrae
appear hollowed out at either end. In such amphicoelous vertebrae a
considerable amount of the chorda always remains, running in an unbroken
string through the whole length of the vertebral column. Towards adult life
the chorda becomes constricted, and is ultimately squeezed out or
destroyed, in the middle of the vertebra, by the invasion of cartilage from
the chief arcualia. This intravertebrally situated cartilage has been
described erroneously as chordal cartilage.
The development of the vertebrae proceeds as follows. First appear a pair
of basidorsalia and a pair of basiventralia (Fig. 1, 1, B.D, B.V), blocks
of cartilage, imbedded in and resting upon the thin sheath of the chorda
dorsalis. Next appears a pair of interdorsal blocks, immediately behind the
basidorsals; and somewhat later appears a pair of interventral blocks.
These four pairs of cartilages or "arcualia" each meet, above or below the
chorda, and form semi-rings, which again by extending upwards or downwards
fuse into complete rings, in such a way that the interdorsal and
interventral elements form the intervertebral mass spoken of above. The
basidorsals fuse with the basiventrals, and form the body of the vertebra,
the fusion being effected chiefly by the calcification and ossification of
the lateral connecting portion of the skeletogenous layer. The basidorsalia
form the neural arches with their unpaired short spinous or neural, and the
paired anterior and posterior zygapophysial processes. Concerning the
{13}basiventralia we have to distinguish between the trunk and the tail. In
the latter they produce a pair of ventral outgrowths or haemapophyses,
which ultimately enclose the caudal blood-vessels. In the trunk the
basiventral blocks of cartilage are suppressed; they appear in the early
larvae, but disappear during or even before metamorphosis.
[Illustration: FIG. 1.–1-5, Five successive stages of the development of a
caudal vertebra of a newt; 6-7, the second and the first cervical vertebra
of _Cryptobranchus_; 8-9, side view of the constituent cartilaginous blocks
of a caudal vertebra (8) and a trunk-vertebra (9) of _Archegosaurus_ as
typical examples of Temnospondylous quadripartite and tripartite vertebrae.
The cross-hatched parts indicate the articular facets for the ribs. The
anterior end of all the vertebrae looks towards the right side. _af_, In 7,
articulating facet for the occipital condyle; _B.D_, basidorsal piece or
neural arch; _B.V_, basiventral piece or ventral arch; _Ch_, chorda
dorsalis, or notochord; _I.D_, interdorsal piece; _I.V_, interventral
piece; _I.V.L_, intervertebral ligament; _N_, spinal nerve–these are
numbered I, II, III in 6 and 7; _R_, rib; _T_, in 7, rib-like tubercle on
the first vertebra.]
Towards the end of the tail the vertebrae diminish in size, and their
constituent cartilages assume a more and more indifferent shape, until they
become confluent into a continuous rod of cartilage, resembling in this
respect the Dipnoi and Holocephali. A periodical revival of this rod, at
least of its connective tissue, appears in the tail-filament of the male
_Triton palmatus_ during the breeding-season.
The first vertebra, called the atlas, because it carries the head, is
remarkable for the possession of an odontoid process. The latter is formed
by a pair of cartilages and represents part of a vertebra, the dorsal
portion of which seems to have been added to the occipital part of the
cranium.
{14}All the trunk-vertebrae, with the exception of the atlas, carry ribs,
at least vestiges thereof. Owing to the early disappearance of the
basiventral cartilages the capitular portions of the ribs are much reduced,
and are mostly represented by strands of connective tissue only. The ribs
develop therefore occasionally at some distance from the vertebral column,
and that portion of the rib which in the metamorphosed young newt looks
like the capitulum is to a great extent really its tuberculum. Witness the
position of the vertebral artery, which still indicates the true foramen
transversarium. The homologies of these parts are still more obscured by
the fact that a new process grows out from the rib, by which the latter
gains a new support upon a knob of the neural arch. Thus an additional
foramen is formed, sometimes confounded with the true transverse canal. The
meaning which underlies all these modifications is the broadening of the
body, the ribs shifting their originally more ventral support towards the
dorsal side. The whole process is intensified in the Anura; it is an
initial stage of the notocentrous type of vertebrae. The transverse
ossified processes of the adult are often much longer than the vestiges of
the ribs themselves, and are somewhat complicated structures. They are
composed first of the rib-bearing cartilaginous outgrowths of the neural
arches; secondly, of a broad string of connective tissue which extends from
the ventro-lateral corner of the perichordal skeletogenous layer to the
ribs.
[Illustration: FIG. 2.–Transverse section through a trunk-vertebra of a
larva of _Salamandra maculosa_, enlarged. The right side shows the actually
existing state, while on the left side the rib and its attachments are
restored to their probable original condition. _A_, Vertebral artery within
the true transverse canal; _B.V_, remnant of the basi-ventral cartilage;
_Ch_, chorda dorsalis; _Sp.c_, spinal canal; *, the false transverse
canal.]
The SHOULDER-GIRDLE is extremely simple. It remains almost entirely
cartilaginous, and the three constituent elements are not separated by
sutures. Ossification is restricted to the base of the shaft of the
scapula, and may extend thence over the glenoid cavity. The coracoids are
broad, loosely overlap each other, and are "tenon and mortised" into the
triangular or lozenge-shaped {15}cartilaginous sternum, which latter has no
connection with the ribs. The precoracoid is a large, flat process,
directed forwards, not meeting its fellow; it is absent in _Siren_.
The humerus articulates with both radius and ulna, and these two bones of
the forearm remain separate. The elements which compose the wrist and hand
exhibit an almost ideally simple arrangement, slightly varied by the
frequent fusion of two or more neighbouring carpalia into one, and by the
reduction of the number of fingers. Most frequently the intermedium and the
ulnar carpal element fuse together, and there is more often one centrale
instead of two. The wrist and hand of the Urodela represent, however, no
longer the entirely primitive pentadactyle type, owing to the loss of one
finger together with its metacarpal and carpal element. Comparison with the
Anura makes it probable that the Urodela have lost the pollex, their four
fingers being consequently the 2nd, 3rd, 4th, and 5th. _Siren_ has four or
three fingers; _Proteus_ has only three fingers and three large compound
carpal cartilages. In _Amphiuma_, with either three or two fingers, the
ulnare, intermedium, and carpale are fused together, the radiale with the
neighbouring carpale. The number of phalanges in the four-fingered species
is generally 2, 3, 3, 2 respectively.
The PELVIC GIRDLE.–The ilium stands vertically to the vertebral axis,
slanting slightly forwards and downwards. It is attached by means of a rib
to only one vertebra, and this ilio-sacral connection is acetabular in its
position, _i.e._ it lies in the same transverse plane with the acetabulum,
in other words vertically above it. The ventral portion of the pelvis is
formed by one large continuous mass, the united pubo-ischia, the anterior
or pubic portion of which extends forwards in the shape of a broad triangle
(_Necturus_) or as a slender, stalked, Y-shaped cartilage, the epipubis,
which is often movably jointed at its base. The lateral portion of the
pubic cartilage is always perforated by the nervus obturatorius.
Ossification is restricted to the ischium and to the middle of the shaft of
the ilium. The acetabular fossa for the femur is closed. The tibia and
fibula remain separate. The foot is still more primitive than the anterior
extremity, as the majority of Urodela possess the full complement of five
toes, with 2, 2, 3, 3, 2 phalanges respectively. Concrescence of the
tarsalia applies most frequently to the fourth and fifth distal {16}and to
the two centralia; exceptional, for instance, in _Cryptobranchus
japonicus_, are as many as three centralia, but this is an individual, even
a one-sided variation, as shown for instance by a specimen in the Cambridge
Museum. Loss of the fifth toe occurs sporadically in genera of different
groups, namely, in _Salamandrella_, _Batrachyperus_, _Salamandrina_,
_Necturus_, _Manculus_, _Batrachoseps_. In _Amphiuma_ the number is reduced
to three or two; in _Proteus_ to two; and in _Siren_ the hind limbs, with
their girdle, are altogether absent. Lastly, in some species of _Spelerpes_
and _Batrachoseps_ both fore and hind limbs have become so small as to be
practically without function, parallel cases being found among various
Scincidae and other Lizards.
The HYOID APPARATUS is still very primitive in many, especially in larval,
Urodela. Besides the hyoid there are as many as four pairs of branchial
arches, which, however, decrease in size and completeness, so that the last
two have lost their connection with the median copular piece, and become
attached in various ways to the second branchial arch. This is the
arrangement apparently in all larvae, but four pairs of branchials persist
in the adult _Siren_, _Amphiuma_, and _Cryptobranchus alleghaniensis_. The
whole branchial apparatus is reduced to three pairs of arches in _Necturus_
and _Proteus_, to two in the adult _Cryptobranchus japonicus_ and in the
Salamandridae. Of considerable interest is the vestige of a fifth pair of
arches in the larvae of _Triton_ and _Salamandra_, in the shape of a pair
of tiny cartilages, which lie in front and on each side of the opening of
the trachea, and give rise to the formation of the laryngeal cartilages,
better developed in the higher Vertebrata.
The following are noteworthy characters of the SKULL OF URODELA. The
articulation of the skull with the vertebral column is not always effected
entirely by the two condyles of the lateral occipital bones, but the median
basal cartilage often possesses a pair of facets for the odontoid-like
process of the first vertebra; such additional facets are perhaps best
developed in _Cryptobranchus_ and in the Salamandrinae.
The middle portion of the primitive cranium, from the exit of the optic
nerve to the ethmoid cartilage, is formed by a pair of separate bones, the
orbito-sphenoids. The parietal and frontal bones remain separate. One or
more periotic bones exist, besides the prootic, in the aquatic families.
{17}A pair of prefrontal bones is present in most Salamandridae, e.g.
_Salamandra_, _Triton_, _Amblystoma_, especially in the larva, and in
_Cryptobranchus_; these bones are absent in _Amphiuma_, _Necturus_,
_Proteus_, and _Siren_.
[Illustration: FIG. 3.–Skulls of various Urodela. 1, _Salamandra maculosa_,
ventral view, and 2, dorsal view; 3, Axolotl stage of _Amblystoma_; 4,
adult stage of _Amblystoma_; 5, _Salamandrina perspicillata_ (after
Wiedersheim); 6, _Salamandra maculosa_, dorsal view of the lower jaw. _A_,
Articulare; _C_{1}_, _C_{2}_, outer and inner occipital condyles; _Ch_,
choana or posterior nasal opening; _d_, dentary; _E_, ethmoid; _F_,
frontal; _LO_, lateral occipital; _M_, maxillary; _N_, nasal; _No_,
nostril; _OS_, orbito-sphenoid; _P_, parietal; _Pf_, prefrontal; _Pl_,
palatine; _Pm_, premaxillary; _Po_, prootic; _PS_, parasphenoid; _Pt_,
pterygoid; _Q_, quadrate; _S_, angulo-splenial; _Sq_, squamosal; _St_,
stapes; _Vo_, vomer; II, VII, X, exits of the optic, facial, and
glosso-vagus nerves.]
The lacrymalia are still separate in some Amblystomatinae, e.g. _Ranidens_
and _Hynobius_. A pair of nasalia are generally present, but are absent in
_Necturus_, _Proteus_, and _Siren_. The parasphenoid is furnished with
teeth in the Plethodontinae and Desmognathinae.
Separate palatine bones exist in _Necturus_ and _Proteus_, and in the larva
of _Amblystoma_, but in the adult form they fuse with the vomers, producing
the vomero-palatines characteristic of the majority of Urodela.
{18}The pterygoid bones are most fully developed, so as to reach the
vomero-palatines, in the Amblystomatinae, in _Necturus_, and in _Proteus_;
they are reduced, so as to leave a gap, in _Cryptobranchus_, and still more
in the Salamandrinae; they are absent in _Amphiuma_ and in _Siren_.
The quadrates are directed forwards in _Necturus_, _Proteus_, and _Siren_,
while in the other Urodela they extend transversely and almost
horizontally. The hyomandibular remnant, the so-called operculum, is small,
and forms a plate which fits into the fenestra ovalis, extending as a
ligamentous process upon the quadrate.
The quadrato-jugal elements are reduced to ligaments. In many Salamandrinae
the large orbito-temporal space is divided into an orbital and a temporal
fossa by an arch which is formed by the meeting of two corresponding
processes from the squamosal and frontal bones respectively. This bridge is
rarely bony (_Salamandrina_, _Triton_), mostly ligamentous;–apparently a
reminiscence of the Stegocephalous condition. The two premaxillary bones
are liable to fuse into one, for instance in _Cryptobranchus_, generally in
adult Tritons. They are most reduced, and are toothless, in _Siren_.
The two maxillary bones are absent only in _Necturus_, _Proteus_,
_Typhlomolge_, and _Siren_. Their posterior end is frequently free, loosely
connected by ligaments with the pterygoid in _Cryptobranchus_; or with the
distal portion of the quadrate, and in this case either just touching it
(_Tylototriton_), or forming a broad junction (_Pachytriton_).
Each half of the lower jaw consists of a dentary, articular and
angulo-splenial. The splenial remains as a separate element in _Siren_; in
others only during the larval period. There are no mento-Meckelian
elements.
SKELETON OF THE ANURA
THE VERTEBRAL COLUMN.–The distinctive peculiarities of the vertebrae of the
Anura are that they are notocentrous, and that about a dozen of them are
modified and fused into an os coccygeum. The whole column is the most
specialised found in the Vertebrata; and various stages are rapidly hurried
through and obscured caenogenetically during the embryonic development.
Paired cartilages appear on the dorsal side of the thin chordal sheath, and
whilst tending to enclose the spinal cord in a {19}canal, their bases grow
head- and tail-wards into what will ultimately become the intervertebral
region. This extension of cartilage leads to a fusion with that of the next
following pair of arches, so that the axial column at this early stage
consists of a right and left longitudinal ridge of cartilage which sends
off dorsal processes, neural arches, in metameric succession. Next, the
intervertebral cartilage increases in such a way as to constrict the chorda
either laterally (_Rana_) or obliquely from above downwards and inwards
(_Bufo_, _Hyla_). We recognise in this cartilage the interdorsalia. Ventral
arcualia are late and much obscured. There is scarcely any cartilage which
could represent the interventralia, the intervertebral cartilage being
almost entirely made up of the interdorsalia. These fuse together and form
a disc or nodule, which later fuses either with the vertebra in front, and
in this case fits into a cup carried by the vertebra next behind
(procoelous vertebrae), or the knob is added to the front end of the
vertebra, fitting into a cup formed by the tail end of the vertebra next in
front (opisthocoelous vertebrae). Much later than the two longitudinal
dorsal bands there appears on the ventral side an unpaired band in which
appear metamerically repeated swellings of cartilage, likewise unpaired.
These swellings become confluent, in a way similar to that which produced
the dorsal bands, and form the unpaired ventral band of cartilage, the
hypochordal cartilage of some authors. The swellings in this band,
equivalent to the basiventralia, become semilunar in a transverse view,
their horns tending upwards towards the basidorsal cartilages, but there is
no actual meeting. Both dorsal and ventral elements are, however, joined
together and form the chief portion of the vertebrae, owing to the rapidly
proceeding calcification and later ossification of the all-surrounding
"membrana reuniens" or skeletogenous layer so far as that is not
cartilaginous.
Procoelous vertebrae exist in the overwhelming majority of Anura;
opisthocoelous are those of the Aglossa, the Discoglossidae, and of some
Pelobatidae. The systematic value of this pro- or opistho-coelous character
has been much exaggerated. We have seen that the centra of the vertebrae of
the Anura are formed entirely by the interdorsal elements, hence the term
"notocentrous," and these centra sometimes remain in adult specimens of
_Pelobates_ as separately ossified and calcified pieces, {20}not fused with
the rest of the vertebrae. This important discovery has been made by
Boulenger, but Stannius had previously mentioned a specimen of _Pelobates_
in which the second and fourth vertebrae are biconvex, the third, sixth,
and eighth biconcave. Moreover, since the sacral vertebra, generally the
ninth, in all the Anura is invariably biconvex, the eighth being biconcave
in the procoelous families, opisthocoelous like the remaining seven
vertebrae in the other families, it is not difficult to imagine that in the
Anura the production of pro- or opistho-coelous vertebrae depends simply
upon the centra or articulating knobs happening to fuse either with the
hind or the front end of the vertebrae. This must of course ultimately be
determined by a mechanical problem of motion.
A second type of the vertebrae amongst the Anura is the epichordal type, an
exaggeration in degree of the notocentrous tendencies of the more usual
perichordal arrangement. It shows, namely, the almost complete suppression
of all the ventral cartilaginous elements, so that the chorda remains for a
long time on the ventral surface of the axial column in the shape of a
flattened longitudinal band. These two types are not unconnected. The
suppression of the ventral elements applies most typically to the trunk
region, while hypochordal cartilage exists in the anterior cervical
vertebrae, and above all in the coccyx. Typically epichordal are the
vertebrae of _Pipa_, _Xenopus_, _Bombinator_, _Pelobates_, _Discoglossus_
and _Alytes_. It is significant that the epichordal often coincide with
opisthocoelous vertebrae, and still more suggestive is the fact that
_Bombinator_ is eminently aquatic, _Pipa_ and _Xenopus_ entirely so, having
lost the tympanum, at least externally. The epichordal feature is not
necessarily indicative of relationship. It has probably been developed
independently in various groups, in correlation with a resumption of
aquatic life. Various genera of Pelobatidae and most likely some
Cystignathidae, e.g. _Pseudis_, will not improbably connect the two types
and their several correlated features, for instance, the frequent reduction
of the tympanic cavity.
The os coccygeum has retained rather primitive features in so far as much
dorsal and ventral cartilage is developed; but this has almost entirely
lost its metameric arrangement, and the posterior half of the coccyx is
formed chiefly by the ventral mass of cartilage, while the dorsal elements
are more or less reduced. {21}Only two vertebrae, generally the tenth and
eleventh of the whole column, are clearly visible, each being composed of a
pair of dorsal and a pair of ventral cartilaginous blocks. The sacral
vertebra articulates with the coccyx by one or two convexities, but in the
Aglossa, in some Pelobatidae, and a few others, the coccyx is fused with
the sacral vertebra. Beyond the first and second component vertebrae of the
embryonic coccyx, the cartilage is continued in the shape of two dorsal,
and one ventral, bands, which soon fuse with each other. Dorsally this
cartilage surrounds the spinal cord; the latter degenerates towards the end
of the tadpole-stage, leaving, however, the empty spinal canal. The chorda,
completely surrounded by cartilage, persists into the post-larval stage,
but is destroyed long before the creature attains maturity. Ultimately the
whole coccyx ossifies.
The tail proper, namely that portion which is absorbed during the
metamorphosis, remains throughout its existence in an apparently primitive
condition. The chorda dorsalis and the spinal cord extend through its whole
length, surrounded by continuous connective tissue without any cartilage;
in fact it represents a piece of typical vertebral column before the
appearance of cartilage. The reduction of this swimming organ begins at the
hind end.
The vertebral column of the adult.–The first vertebra (we will call it the
atlas since it carries the skull) is not, as in the Urodela, provided with
an odontoid process. It articulates by two cups with the condyles of the
occiput. In some Anura it co-ossifies, rather incompletely, with the second
vertebra, regularly in the fossil _Palaeobatrachus_, often in
_Ceratophrys_, _Breviceps_, and occasionally in _Pelobates_, _Bufo_,
_Rana_, and _Xenopus_. This is, however, no justification for looking upon
the first vertebra as a complex of two vertebrae, although the atlas is
frequently very thick and broad, and even carries, in the Aglossa,
considerable lateral wings or diapophyses. Those of the trunk-vertebrae are
often very long, acting thereby as substitutes for ribs which are absent,
except on the second, third, and fourth vertebrae of the Discoglossidae,
and on the second and third of the Aglossa. In the adult Aglossa these ribs
fuse with the processes which carry them.
The diapophyses of the sacral vertebra carry no ribs, the ilia being
attached to them directly. They are either cylindrical {22}as in the
Ranidae and Cystignathidae, or they are more or less dilated as in all the
other families, most strongly in the Pelobatidae and the Aglossa. In some
members of the large sub-family of the Cystignathidae the otherwise
cylindrical diapophyses are slightly dilated.
[Illustration: FIG. 4.–Dorsal view of the sacral or ninth vertebra (9),
with the attachment of the ilium, of (1) _Rana temporaria_, (2) _Bufo
vulgaris_, showing the whole coccyx and pelvis, (3) _Pelobates fuscus_, as
examples of cylindrical and of dilated sacral diapophyses. (About nat.
size.) _a_, Acetabulum; _c_, coccyx; _i_, ilium; _z_, anterior
zygapophyses.]
The sacrum is formed by the ninth vertebra, but there are a few interesting
exceptions. _Pelobates_, _Pipa_, and _Hymenochirus_ possess two sacral
vertebrae; and, neglecting individual abnormalities, these three genera
form the only exception amongst recent Amphibia. In the three genera the
coccyx is fused with the second sacral vertebra, and such a fusion occurs
elsewhere normally only in _Bombinator_ with its single sacral vertebra.
The morphologically oldest condition is normally represented by
_Pelobates_, the sacral vertebrae being the tenth and ninth. One case has
been recorded by Boulenger of _Bombinator pachypus_ "with eleven segments,"
the last carrying the ilium. Individual lop-sided abnormalities have been
described in _Bombinator_ and _Alytes_, where the right ilium articulated
with the tenth, the left ilium with the ninth vertebra. This shifting
forwards of the ilium to the extent of one metamere has been continued
further in _Pipa_, in which the sacrum is formed by the ninth and eighth
vertebrae, their diapophyses fusing on either side into extra broad
wing-like expansions. In old specimens of _Palaeobatrachus fritschi_ the
seventh vertebra is in a transitional condition, the ilium being carried by
the ninth and eighth, and slightly also by the diapophyses of the seventh
vertebra; and in _P. diluvianus_ the {23}diapophyses of all these vertebrae
are united into one broad plate to which the ilia are attached. Lastly, in
_Hymenochirus_ the first sacral is the sixth vertebra, and this creature
has thereby reduced the pre-sacral vertebrae to the smallest number known.
This shifting forwards of the iliac attachment implies the conversion of
original trunk into sacral vertebrae, and the original sacral vertebra
itself becomes ultimately added to the urostyle. The second sacral, the
tenth of _Pelobates_, the ninth of _Pipa_, and the tenth on the right side
of the abnormal _Bombinator_, are still in a transitional stage of
conversion. In Discoglossidae the tenth is already a typical post-sacral
vertebra, and is added to the coccyx, but it still retains distinct, though
short, diapophyses. In the majority of the Anura the tenth vertebra has
lost these processes, and its once separate nature is visible in young
specimens only. In _Bombinator_ even the eleventh vertebra is free during
the larval stage. In fact the whole coccyx is the result of the fusion of
about twelve or more vertebrae, which from behind forwards have lost their
individuality. We conclude that originally, in the early Anura, there was
no coccyx, and that the ilium was attached much farther back; and this
condition, and the gradual shifting forwards, supply an intelligible cause
of the formation of an os coccygeum. The fact that the sacral vertebrae of
the Anura possess no traces of ribs as carriers of the ilia, is also very
suggestive. The ilia have shifted into a region, the vertebrae of which had
already lost their ribs. By reconstructing the vertebral column of the
Anura, by dissolving the coccyx into about a dozen vertebrae, so that
originally, say the twenty-first vertebra carried the ilia, we bridge over
the enormous gap which exists between the Anura and Urodela. That whole
portion of the axial continuation behind the coccyx, more or less
coinciding with the position of the vent, is the transitional tail.
The disappearance of both notochord and spinal cord, and the conversion of
the cartilaginous elements into a continuous rod in the case of the os
coccygeum, find an analogy in the hinder portion of the tail of Dipnoi and
Crossopterygii, and in the tail-end of most Urodela, portions which are not
homologous with the os coccygeum. The term urostyle should be restricted to
such and similar modifications of the tail-end, and this latter happens to
be lost by the Anura during metamorphosis.
{24}Strictly speaking, or rather in anatomical parlance, the Vertebrate
tail begins with the first post-sacral vertebra. In the Anura that portion
of the whole tail has retained most cartilage, and has become the
coccygeum, which is required as a "backbone" for the often enormous belly.
This requirement is an outcome of the great shortening of the trunk proper
(if the trunk be defined as ending with the pelvic region), and this
shortening of the trunk is again intimately connected with the jumping
mechanism, enlargement of the hind-limbs, elongation of the ilia, and
throwing the fulcral attachment forwards as much as possible. The
pre-acetabular ilio-sacral connection is carried to the extreme in the
Anura.
The SHOULDER-GIRDLE and "sternum" are more complete than in the Urodela,
there being also a pair of clavicles, fused with the precoracoidal bars.
The whole apparatus presents two types. In the arciferous type the
coracoids and precoracoids retain a great amount of cartilage in their
distal portions, and these cartilages (the epicoracoids of some authors)
overlap each other movably on one another, the right usually lying
ventrally upon the left. The epicoracoidal cartilage of each side, by
connecting the distal end of the coracoid with the precoracoid of the same
side, forms an arc, hence "arciferous." In the firmisternal type the
epicoracoidal cartilages are much reduced, and, instead of overlapping,
meet in the middle line and often fuse with each other, forming thereby a
firm median bar, which connects the ventral ends of the precoracoids with
those of the coracoids. This type is morphologically the higher and more
recent, and passes in the larval stage through the arciferous condition. It
is restricted to the Ranidae, Engystomatinae, and Aglossa. Although these
two types afford an excellent distinctive character for the main divisions
of the Anura, they are to a certain extent connected by intermediate forms
in such a way, that, for instance, in _Bufo_ and among Cystignathidae in
_Ceratophrys_, the two opposite epicoracoidal cartilages begin to unite at
the anterior end.
In many Engystomatinae the precoracoids together with the clavicles are
much reduced, sometimes to thin ligaments, being in this case mostly curved
back and lying closely against the coracoids; or they may be lost
completely. Very rarely the precoracoidal bars are actually much stronger
than the coracoids, {25}and the median symphysial bar of cartilage is lost;
this is the case in _Hemisus_.
The scapula is always large and curved into transverse, dorsally broadening
blades, the dorsal greater portion of which, the so-called supra-scapula,
does not ossify but calcifies.
[Illustration: FIG. 5.–Ventral views of the shoulder-girdles of various
Anura. (Slightly enlarged.) 1, _Bombinator igneus_, and 2, _Bufo vulgaris_,
as examples of the arciferous type; 3, adult, 4, metamorphosing _Rana
temporaria_ showing change from the arciferous into the firmisternal type;
5, _Hemisus guttatum_; 6, _Breviceps gibbosus_; 7, _Cacopus systoma_. (5,
6, 7, after Boulenger.) Cartilaginous parts are dotted; ossified parts are
left white. _Cl_, Clavicle; _Co_, coracoid; _E_, epicoracoidal cartilage;
_H_, humerus; _M_, metasternum; _O_, omosternum; _P_, precoracoid; _Sc_,
scapula; _S.S_, supra-scapula.]
It is very doubtful if the Anura possess a true sternum, if by sternum we
understand a medio-ventral apparatus which owes its origin to the ventral
portions of ribs. The so-called sternal apparatus of the Anura consists of
two pieces. One, anterior, variously named episternum, presternum, or
omosternum, rests upon the united precoracoids and extends headwards, being
either styliform or broadened out. Sometimes it is partly ossified, with a
distinct suture at its base; this is the case especially in the
Firmisternia; in many Arcifera the omosternum remains cartilaginous and is
continuous, without a sutural break, with the cartilage of the
precoracoids, indicating thereby its genetic relation to the
shoulder-girdle. Hence _omosternum_ is the {26}preferable name. It is
frequently much reduced, even absent, for instance in most Bufonidae and in
the Engystomatinae. The posterior so-called sternal part may be termed
_metasternum_. It forms the posterior counterpart of the omosternum. It is
attached behind to the epicoracoidal cartilages, or fusing with them forms
their posterior continuation. It appears mostly in the shape of a style,
which is frequently ossified, and broadens out behind into a cartilaginous,
partly calcified blade. In the Discoglossidae only it diverges backwards
into two horns, assuming a striking resemblance to the typical xiphisternum
of the Amniota. In young Anura the metasternal cartilage is intimately
connected with the pericardium, an indication of its being derived not from
ribs but from the shoulder-girdle.
The glenoid cavity is always formed by the coracoids and by the scapula,
but the precoracoid often takes part in its formation, for instance in
Bufonidae, Hylidae, and Discoglossidae.
In the FORE-LIMB the humerus has a crest, stronger in the males than in the
females; it assumes extraordinary strength in some Cystignathidae, notably
in the male _Leptodactylus_. Radius and ulna are fused into one bone. The
carpalia are originally nine in number: radiale, ulnare, two centralia, and
five carpalia distalia, the fifth of which is reduced to a tiny nodule or
to a ligamentous vestige. The primitive condition still prevails in the
Discoglossidae. In most of the other Anura the fourth and third distal
carpalia, in any case very small, fuse with the enlarged ulnar centrale;
the radial centrale comes, in the Bufonidae and Pelobatidae, into contact
with the radius, so that the forearm articulates with three elements as in
the Urodela, but with this difference, that the intermedium of the Urodela
has been lost by the Anura. There are five metacarpalia and five fingers,
but the elements of the first or thumb are nearly vestigial, so that the
pollux is reduced to one or two nodules, scarcely visible externally. The
normal number of the phalanges of the second to fifth fingers is 2, 2, 3,
3. The distal phalanges are generally straight, either pointed or expanded
or with Y or T-shaped ends; but in the Hylidae, in _Hylambates_ amongst the
Ranidae, and in _Ceratohyla_, one of the Hemiphractinae, the terminal
phalanges are produced into curved claws which support the adhesive
finger-discs. There are, however, many genera of different families, which
possess finger-discs and have no claw-shaped {27}phalanges. The Hylidae,
and many of the climbing members of the Ranidae with adhesive discs,
possess an extra skeletal piece intercalated between the last and last but
one phalanges of the fingers and toes. This piece, a mere interarticular
cartilage in _Hyla_, is in the following Raninae developed into an
additional phalanx, so that their numbers are 3, 3, 4, 4 in the hand and 3,
3, 4, 5, 4 in the foot: _Cassina_, _Hylambates_, _Rappia_, _Megalixalus_,
_Rhacophorus_, _Chiromantis_, _Ixalus_, and _Nyctixalus_. All the other
Ranidae are without this additional phalanx, irrespective of the presence
or absence or size of digital expansions.[12]
The PELVIC GIRDLE looks like a pair of tongs (see Fig. 4, p. 22). The ilium
is enormously elongated and is movably attached to the sacral diapophyses.
This connection is always pre-acetabular in position. The ilium and ischium
co-ossify completely, and make up nearly the whole of the pelvis; the pubis
is very small, and remains cartilaginous unless it calcifies. It rarely
possesses a centre of ossification, for instance in _Pelobates_, where the
osseous nodule is excluded from the acetabulum, recalling certain
Labyrinthodonta, whose ossa pubis likewise do not reach that cavity. The
latter is open or perforated in young Anura and remains so in the
Discoglossidae, but in the others it becomes closed up as in the Urodela.
The ventral halves of the pelvis, besides forming a symphysis, closely
approach each other, just leaving room for the passage of the rectum and
the urino-genital ducts.
The HIND-LIMBS are in all cases longer than the fore-limbs. The femur is
slender, the tibia and fibula are fused into one bone. The tarsus is much
modified by the great elongation of the two proximal tarsalia (there being
no intermedium) into an astragalus and a calcaneum, both of which fuse
together distally and proximally, or completely as in _Pelodytes_; in the
latter case the limb assumes a unique appearance, since it consists of
three successive and apparently single bars of nearly equal length. The
other tarsal elements, especially the more lateral ones, are practically
reduced to pads. The Anura have thereby acquired two well-marked joints,
one cruro-tarsal, the other tarso-metatarsal; this shows a high stage of
specialisation in comparison with the Urodelous and Stegocephalous type of
still undefined joints.
{28}The Anura possesses five well-developed toes with normally 2, 2, 3, 4,
and 3 phalanges, and the rudiments of a sixth digit, the so-called
prehallux, which consists of from two to four pieces, including the one
which represents its metatarsal. This prehallux, as a vestige of a once
better developed digit, is exactly like the elements on the radial side of
the wrist, which, we are certain, are the remnants of a once complete
finger, namely the pollex. The only weighty difficulty against its
interpretation as a prehallux lies in the fact that hitherto no six-toed
Stegocephali have been found; but the fact that there are no Stegocephali
known with more than four fingers could be used as an argument against
there being a pollex-vestige in recent Anura with just as little reason.
The SKULL of the Anura differs from that of the other recent Amphibia in
the following features:–
The orbital region of the primitive cranium remains cartilaginous, but
further forward the cranial cavity is closed by the unpaired sphenethmoid,
which forms a ring round the anterior portion of the brain-cavity, hence
called "os en ceinture" by some anatomists. The frontals and parietals fuse
into one pair of fronto-parietal bones, and these again can fuse together
in the middle line; as in Aglossa and _Pelobates_. The palatal portion of
the palato-quadrate cartilage is complete, reaching forwards to the sides
of the ethmoid region. The curved arch, formed by this cartilage, is
covered by the following bones: (1) the quadrato-jugal, reduced to a thin
splint which connects the quadrate and squamosal with the posterior end of
the maxilla; (2) the pterygoid, always strong, extending from the distal
inner corner of the quadrate to the maxilla, sometimes also to the
palatine, and with a broad, median process to the parasphenoid, this
process covering ventrally most of the otic region; (3) the palatines,
which vary considerably in shape and size; they are placed transversely and
meet in the middle line; in _Bombinator_ and _Pelodytes_ they are absent.
The quadrates are directed transversely and backwards, in conformity with
the wide gape of the mouth. The squamosal is always well developed,
covering the whole of the quadrate on its outer side; it has a forwardly
directed process which ends freely in _Rana_, meets a corresponding process
of the maxilla and forms a bony arch with it in _Discoglossus_,
_Pelobates_, and others, or {29}is scarcely developed at all, for instance
in _Bufo_. In _Pelobates cultripes_ the squamosal is very wide and forms a
junction with the fronto-parietals, thus producing a broad bridge across
the temporal fossa.
The nasal bones are large and meet in the middle line. Frequently they
leave a space between them and the diverging anterior portion of the
fronto-parietals, through which gap appears part of the dorsal surface of
the ethmoid cartilage. A fontanelle between the frontals occurs in most
Hylidae, many Cystignathidae, some few Bufonidae, in _Pelodytes_ amongst
the Pelobatidae, and in the Discoglossidae.
The tympanic cavity is bordered in front, above, and below by the squamosal
and quadrate, behind by the musculus depressor mandibulae, internally by
the otic capsule, and by the cartilage of the cranium between this and the
lateral occipital bone. The cavity communicates, however, by the wide and
short Eustachian tube with the mouth, the passage being bordered anteriorly
by the pterygoid, posteriorly by soft parts. Partly imbedded in these soft
tissues is the styloid process or stylohyal, which is attached to the
cranium, mostly behind the otic region, and is continued downwards into the
anterior horn of the hyoid. The whole partly cartilaginous, ligamentous,
and osseous string is, in fact, the entire ventral half of the hyoid arch,
while the dorsal half or hyomandibular portion of this, the second visceral
arch, is modified into the columellar or auditory chain. The inner end of
this chain, the stapes, is inserted into and around the fenestra ovalis of
the otic capsule, while the outer end is somewhat T-shaped, and is loosely
attached to or near the upper rim of the tympanic ring and to the middle of
the tympanic disc. In many Anura this terminal bar can be seen from the
outside. The middle portion of the columellar chain is ossified, the rest
remains cartilaginous. But the whole chain exhibits various modifications
in different genera, especially in the number and the extent of the
processes sent out by the outer cartilaginous portion; these are attached
in various ways to the tympanum and its rims. The tympanic disc is carried
by a cartilaginous ring, which rests against a special process sent out by
the quadrate, and is probably itself a differentiation of this element.
In some very aquatic genera, but also in _Pelobates_, the {30}tympanic
cavity is much reduced, for instance in _Bombinator_, _Liopelma_. In
_Batrachophrynus_ not only the cavity, but also the Eustachian tubes are
suppressed. In the Aglossa only the two tubes are united into one short but
wide median canal, opening at the level of the pterygoids on the roof of
the mouth.
The lower jaw is remarkable for the possession of mento-Meckelian
cartilages, absent only in the Aglossa and Discoglossidae. At first they
are much longer than the rest of the jaw; during the larval life they
indeed form the functional jaw, and they are now covered with horny sheaths
instead of teeth. Owing to the absence of teeth on them, these
mento-Meckelian cartilages are later not invested by bone, although in many
Anura they ultimately ossify, either retaining their separate nature or
fusing partly with the dentary bones. The bulk of the lower jaw, the
Meckelian cartilage, becomes invested by the dentary, a small articulare,
and an inner angulare, while a splenial element is absent. The dentary
itself is mostly reduced to a small dentigerous splint, while the angulare
forms by far the greater part of the bony jaw.
Teeth are more restricted in their occurrence than in the Urodela. On the
jaws they always stand in one row. With the exception of the
Hemiphractinae, Amphignathodontinae, Ceratobatrachinae, and Genyophryninae,
no recent Anura carry teeth on the lower jaw, and even in these genera they
are mostly much reduced in size and firmness, having all the appearance of
vanishing structures. The premaxillae and maxillae are frequently furnished
with teeth, except in the Dendrobatinae, Genyophryninae, Engystomatinae,
Dendrophryniscinae, Bufonidae, _Pipa_, and _Hymenochirus_. The vomers
mostly carry a series of teeth on their posterior border; when these teeth
are absent, as in many species of _Bufo_, a kind of substitute sometimes
occurs on the palatines in the shape of a row of tuberosities. The
palatines carry teeth in Hemiphractinae. The parasphenoids are rarely
toothed, _e.g._ _Triprion_, _Diaglena_, _Amphodus_, and occasionally in
_Pelobates_.
A few Anura possess peculiar substitutes for teeth in the anterior portion
of the lower jaw, namely, a pair of conical bony processes, sometimes
rather long, but always covered by the dense gums, or investment of the
jaws; _e.g._ _Lepidobatrachus_, several Rana, _e.g._ _R. adspersa_, _R.
khasiana_, _R. kuhli_, and _Cryptotis brevis_.
{31}Cranial dermal ossifications are developed in some species of _Bufo_,
still more in the Hemiphractinae, and above all in _Pelobates cultripes_
and in the Cystignathoid genus _Calyptocephalus_.
The HYOID APPARATUS of the Anura is complicated. It is originally composed
of the hyoidean and four branchial arches, with one median, copular piece.
The branchial arches form in the early life of the tadpole the elaborate
framework of the filtering apparatus mentioned on p. 44. During
metamorphosis the whole filter disappears, owing to resorption of the
greater part of the branchial arches; only their median portions remain,
and fuse with the enlarged copular piece and the hyoidean arches into a
broad shield-shaped cartilage (_corpus linguae_), whence several lateral
processes sprout out, the posterior pair of which are generally called
thyrohyals or thyroid horns. The true hyoid horns give up their larval
lean-to articulation with the quadrate, become greatly elongated, and gain
a new attachment on the otic region of the cranium. The transformation of
the whole apparatus has been studied minutely by Ridewood, in _Pelodytes
punctatus_.[13]
SKIN
The epidermis of the young larvae of Amphibia is furnished with cilia,
which later on are suppressed by the development of a thin hyaline layer or
cuticula, but clusters of such cilia remain, at least during the larval
life and during the periodical aquatic life of the adult, in the epidermal
sense-organs. In the frog, currents are set up by the ciliary action at an
earlier stage, and are maintained to a later stage than in the newt. In the
latter the tail loses its ciliation, whereas in the frog it remains active
almost up to the time of the metamorphosis. In tadpoles of 3-10 mm. nearly
the whole surface is ciliated (Assheton).[14] The cilia work from head to
tail, causing the little animal, when perfectly quiet, to move forwards
slowly in the water. Beneath the cuticula, in the Perennibranchiata and the
larvae of the other Urodela, lies a somewhat thicker layer of vertically
striated cells, the so-called pseudo-cuticula, which disappears with the
transformation of the upper layers of the Malpighian cells into the stratum
corneum. The latter is very thin, consists of one or two layers of
flattened cells, and is shed periodically by all {32}Amphibia in one piece.
In the Urodela it generally breaks loose around the mouth, and the animal
slips out of the delicate, transparent, colourless "shirt," which during
this process of ecdysis or moulting becomes inverted. In the Anura it
mostly breaks along the middle line of the back, the creature struggles out
of it, pokes it into its mouth, and swallows it. Urodela also eat this
skin. As a rule the first ecdysis takes place towards the end of the
metamorphosis, preparatory to terrestrial life. So long as the animal grows
rapidly, the skin has to be shed frequently, since this corneous layer is
practically dead and unyielding. Adult terrestrial Urodela do not seem to
moult often, mostly only when they take to the water in the breeding
season. Anura, on the other hand, moult often on land, at least every few
months. The surface of the new skin is then quite moist and slimy, but it
soon dries and hardens.
The Malpighian stratum consists of several layers, thickest in the
Perennibranchiata; in them it contains mucous cells throughout life, in
others such slime-cells are restricted to larval life. Later, regular
slime-glands are developed, which open on the surface. They are very
numerous, and more evenly distributed, over most parts of the body, than
the specific or poison-glands, which are restricted to certain parts, often
forming large clusters, especially on the sides of the body. They reach
their greatest development in the "parotoid glands" of the Anura. Both
kinds of glands are furnished with smooth muscle-fibres, which are said to
arise from the basal membrane underlying and forming part of the Malpighian
layer; these muscle-cells extend later downwards into the corium. For the
action of the poison, see p. 37.
The stratum corneum is mostly thin, but on many parts of the body,
especially in Anura, the epidermal cells proliferate and form hard spikes
or other rugosities, generally stained dark brown. With these may be
grouped the nuptial excrescences so frequent in the Anura, especially on
the rudiment of the thumb, and on the under surface of the joints of the
fingers and toes. In many Anura, less frequently in the Urodela, the tips
of the fingers and toes are encased in thicker horny sheaths, producing
claws or nails. They are best developed among newts in _Onychodactylus_,
among the Anura in _Xenopus_ and _Hymenochirus_. The horny covering of the
metatarsal tubercles reaches its greatest size in {33}the digging spur or
spade of _Pelobates_. In most of these cases the cutis is elevated into
more or less wart-like papillae, covered, of course, by the proliferated
and cornified epidermis. In the female of _Rana temporaria_ nearly the
whole surface of the body becomes covered with rosy papillae during the
breeding season. Similar nuptial excrescences are common, and are most
noteworthy in the male of the Indian _Rana liebigi_.
The epidermis also contains sense-organs. They attain their highest
development in the larvae; later on they undergo a retrogressive change.
Each of these sense-organs is a little cup-shaped papilla, visible to the
naked eye. It is composed of elongated cells which form a mantle around
some central cells, each of which ends in a stiff cilium perforating a
thin, hyaline membrane which lines the bottom of the cup, and is perhaps
the representation of the cuticula. These ciliated cells are connected with
sensory fibres, the nerve entering at the bottom of the whole organ. The
cilia are in direct contact with the water, but the outer rim of the whole
apparatus is protected by a short tube of hyaline cuticula-like secretion.
These sense-organs are, in the larvae, scattered over the head, especially
near the mouth and around the eyes, whence they extend backwards on to the
tail, mostly in three pairs of longitudinal rows, one near the vertebral
column, the others lateral. They are supplied by the lateral branch of the
vagus nerve. They disappear during the metamorphosis, at least in the
Anura, with the exception of _Xenopus_, in which they form conspicuous
white objects. The white colour is caused by the tubes becoming choked with
the débris of cells or coagulating mucous matter, so that it is doubtful if
these organs, which moreover have sunk deeper into the skin, are still
functional. In the terrestrial Urodela these organs undergo a periodical
process of retrogression and rejuvenescence. During the life on land they
shrink and withdraw from the surface, and their nerves likewise diminish,
but in the breeding season, when the newts take again to aquatic life, they
revive, are rebuilt and become prominent on the surface. They are an
inheritance from the fishes, in which such lateral line organs are
universally present.
The cutis of most Amphibia is very rich in lymph-spaces, which, especially
in the Anura, assume enormous proportions, since the so-called subcutaneous
connective tissue forms {34}comparatively few vertical septa by which the
upper and denser layers, the corium proper, are connected with the
underlying muscles. The spaces are filled with lymph, and into some of them
the abnormally expanded vocal sacs extend, notably in _Paludicola_,
_Leptodactylus_, and other Cystignathidae, and in _Rhinoderma_.
The cutis frequently forms papillae and prominent folds, sometimes regular
longitudinal keels on the sides of the back; but dermal, more or less
calcified or ossified scales are restricted to the Stegocephali and to the
Apoda, _q.v._, pp. 79, 87. We conclude that the Urodela and Anura have
entirely lost these organs. Dermal ossifications, besides those which now
form an integral part of the skeleton, like many of the cranial
membrane-bones, are rare, and are restricted to the Anura. They are least
infrequent on the head, where the skin is more or less involved in the
ossification of the underlying membrane-bones, for instance in _Triprion_,
_Calyptocephalus_, _Hemiphractus_ and _Pelobates_. The thick ossifications
in the skin of the back of several species of _Ceratophrys_ are very
exceptional. In _Brachycephalus ephippium_ these dermal bones enter into
connection with the vertebrae; small plates fuse with the dorsal processes
of the first to third vertebrae, while one large and thick plate fuses with
the rest of the dorsal vertebrae. Simple calcareous deposits in the cutis
are less uncommon, for instance, in old specimens of _Bufo vulgaris_. We
are scarcely justified in looking upon these various calcifications and
even ossifications as reminiscences of Stegocephalous conditions.
The skin contains PIGMENT. This is either diffuse or granular. Diffuse
pigment, mostly dark brown or yellow, occurs frequently in the epidermis,
even in the stratum corneum. The granular pigment is stored up in cells,
the chromatophores, which send out amœboid processes, and are restricted to
the cutis, mostly to its upper stratum, where they make their first
appearance. Contraction of the chromatophores withdraws the pigment from
the surface, expansion distributes it more or less equally. The usual
colours of the pigment are black, brown, yellow, and red. Green and blue
are merely subjective colours, due to interference. A peculiar kind of
colouring matter is the white pigment, which probably consists of guanine,
and is likewise deposited within cells; cf. the description of the white
spots in the skin of _Hyla coerulea_.
{35}Most Amphibia are capable of changing colour, the Urodela, however, far
less than the Anura, some of which exhibit an extraordinary range and
adaptability in their changes.
The mechanism by which the change of colour is produced in frogs has been
recently studied by Biedermann.[15] If we examine the green skin of the
common Tree-frog, _Hyla arborea_, under a low power and direct light, we
see a mosaic of green, polygonal areas, separated by dark lines and
interrupted by the openings of the skin-glands. Seen from below the skin
appears black. Under a stronger power the black layer is seen to be
composed of anastomosing and ramified black pigment-cells. Where the light
shines through, the skin appears yellow. The epidermis itself is quite
colourless. The mosaic layer is composed of polygonal interference-cells,
each of which consists of a basal half which is granular and colourless,
while the upper half is made up of yellow drops. Sometimes the tree-frog
appears blackish, and if then the black pigment-cells are induced to
contract, for instance, by warming the frog, it appears silver-grey; in
this case the pigment in the yellow drops is no longer diffuse, but is
concentrated into a round lump lodged between the interstices of the
granular portions; the black pigment-cells are likewise balled together.
These black chromatophores send out numerous fine branches, which
occasionally stretch between and round the polygonal cells. When each of
these is quite surrounded and covered by the black processes, the frog
appears black. On the other hand, when the black pigment-cells withdraw
their processes, shrink up, and, so to speak, retire, then the light which
passes through the yellow drops is, by interference, broken into green.
Stoppage of the circulation of the blood in the skin causes the black
chromatophores to contract. Carbon dioxide paralyses them and causes them
to dilate. This is direct influence without the action of nerves. But
stimulation of the central nerve-centres makes the skin turn pale. Low
temperature causes expansion, high temperature contraction, of the
chromatophores. Hence hibernating frogs are much darker than they are in
the summer. Frogs kept in dry moss, or such as have escaped into the room
and dry up, turn pale, regardless of light or darkness, probably owing to a
central, reflex, nerve-stimulus.
Tree-frogs turn green as the result of the contact with leaves. {36}Dark
frogs will turn green when put into an absolutely dark vessel in which
there are leaves. This is reflex action, and blinded specimens do the same.
The principal centres of the nerves which control the chromatophores, lie
in the corpora bigemina and in the optic thalami of the brain. When these
centres are destroyed, the frog no longer changes colour when put upon
leaves, but if a nerve, for instance the sciatic, be stimulated, the
corresponding portion of the body, in this case the leg, turns green. Rough
surfaces cause a sensation which makes the frog turn dark. _Rana_ seems to
depend chiefly upon temperature and the amount of moisture in the air, so
far as its changes of colour are concerned. Biedermann concludes that the
"chromatic function of frogs in general depends chiefly upon the sensory
impressions received by the skin, while that of fishes depends upon the
eye."
All this sounds very well, but the observations and experiments are such as
are usual in physiological laboratories, and the frogs, when observed in
their native haunts, or even when kept under proper conditions, do not
always behave as the physiologist thinks they should. There is no doubt
that in many cases the changes of colour are not voluntary, but reflex
actions. It is quite conceivable that the sensation of sitting on a rough
surface starts a whole train of processes: roughness means bark, bark is
brown, change into brown; but one and the same tree-frog does not always
assume the colour of the bark when it rests, or even sleeps upon, such a
piece. He will, if it suits him, remain grass-green upon a yellow stone, or
on a white window-frame. I purposely describe such conditions, changes,
coincidences, and discrepancies in various species, notably in _Hyla
arborea_, _H. coerulea_, _Rana temporaria_, _Bufo viridis_, to show that in
many cases the creature knows what it is about, and that the eye plays a
very important part in the decision of what colour is to be produced. The
sensory impression received through the skin of the belly is the same, no
matter if the board be painted white, black, or green, and how does it then
come to pass that the frog adjusts its colour to a nicety to the general
hue or tone of its surroundings?
Boulenger[16] has given us a summary of the action of the POISON of
Amphibia:
{37}It is well known to all who have handled freshly-caught newts, and
certain toads, especially _Bombinator_, that their secretion acts as a
sternutatory, and causes irritation of the nose and eyes, the effects
produced on us by _Bombinator_ being comparable to the early stages of a
cold in the head. Many collectors of Batrachians have learned, to their
discomfiture, how the introduction of examples of certain species into the
bag containing the sport of their excursion may cause the death of the
other prisoners; for although the poison has no effect on the skin of
individuals of the same species, different species, however closely allied,
may poison each other by mere contact. But when inoculated the poison acts
even on the same individual.
Miss Ormerod, to personally test the effect, pressed part of the back and
tail of a live Crested Newt between the teeth. "The first effect was a
bitter astringent feeling in the mouth, with irritation of the upper part
of the throat, numbing of the teeth more immediately holding the animal,
and in about a minute from the first touch of the newt a strong flow of
saliva. This was accompanied by much foam and violent spasmodic action,
approaching convulsions, but entirely confined to the mouth itself. The
experiment was immediately followed by headache lasting for some hours,
general discomfort of the system, and half an hour after by slight
shivering fits."
Numerous experiments have shown that the poison of toads, salamanders, and
newts is capable, when injected, of killing mammals, birds, reptiles, and
even fishes, provided, of course, that the dose be proportionate to the
size of the animal. Small birds and lizards succumb as a rule in a few
minutes; guinea-pigs, rabbits, and dogs in less than an hour.
This poison of Amphibia is not septic, but acts upon the heart and the
central nervous system. That of the common toad has been compared, in its
effects, to that of _Digitalis_ and _Erythrophlaeum_. Some authorities hold
that the poison is an acid, others regard it as an alkaloid.
Phisalix[17] has come to the conclusion that toads and salamanders are
possessed of two kinds of glands, different both anatomically and
physiologically. These are, first the mucous glands, spread over the
greater part of the body, with an alkaloid secretion, which acts as a
narcotic; secondly, specific glands, as {38}the parotoids and larger dorsal
glands, the secretion of which is acid, and acts as a convulsive.
The Indians of Colombia are said to employ the secretion of _Dendrobates
tinctorius_ for poisoning their arrows. The poison is obtained by exposing
the frog to a fire, and after being scraped off the back is sufficient for
poisoning fifty arrows. It acts on the central nervous system, and is used
especially for shooting monkeys. Concerning the use of this poison for
"dyeing" parrots, see p. 272.
The milky secretion of toads protects them against many enemies, although
not always against the grass-snake. A dog which has once been induced to
bite a toad, suffers so severely that it will not easily repeat the
experiment. The handling of tree-frogs also irritates both nose and eyes.
The hind limbs of the Water-frog, _Rana esculenta_, have a very bitter,
acrid taste. In short, most, if not all, Amphibia are more or less
poisonous, and it is significant that many of the most poisonous, e.g.
_Salamandra maculosa_, _Bombinator_, _Dendrobates_, exhibit that very
conspicuous combination of yellow or orange upon a dark ground, which is so
widespread a sign of poison. Other instances of such warning colours,
protective in a defensive sense, are the Wasps and _Heloderma_, the only
poisonous lizard.
NERVES
SPINAL NERVES.–Each spinal nerve issues originally immediately behind the
neural arch of the vertebral segment to which it belongs. This
intra-vertebral position is ultimately modified into a more inter-vertebral
one, owing to the predominant share of the neural arches, basidorsalia, in
the composition of the whole vertebra. Consequently the nerves issue behind
their corresponding vertebra.
The first spinal nerve, or N. suboccipitalis, is exceptional in several
respects. It develops a dorsal and a ventral root like a typical spinal
nerve, but the dorsal root soon degenerates in all Amphibia, while in the
Phaneroglossal Anura the whole nerve disappears. The first spinal nerve
reduced to its ventral half persists therefore only in the Apoda, Urodela,
and the Aglossal Anura. It issues originally between the occiput and the
atlas, but in the adult it is partly imbedded in the anterior portion of
the atlas. Its own vertebra is lost, having probably been added to the
cranium.
{39}In the Urodela the first spinal nerve either remains separate, or it
joins the second spinal, forming with it and with a branch from the third
nerve the cervical plexus, which supplies the muscles of the cervical
region. The third, fourth, and fifth nerves, and sometimes also the sixth,
form the brachial plexus.
In the Aglossal Anura N. spinalis I. mostly sends a fine thread to the
second spinal nerve, the rest supplies chiefly the M. levator scapulae, in
_Pipa_ the abdominal muscles also. In all the other Anura this N. spinalis
I. is lost; occasional vestiges have been reported in _Bufo vulgaris_ and
_Rana catesbiana_, and remnants of it may possibly be found in Pelobatidae
and Discoglossidae. The first actually persisting nerve of the
Phaneroglossa is consequently N. spinalis II.
The brachial plexus is composed as follows:–_Pipa_, N. spinalis II. and
III.; _Xenopus_ and Phaneroglossa, N. spinalis III. and IV., with a small
branch from the second; the next following three nerves, numbers V., VI.,
and VII., behave like ordinary trunk nerves.
The pelvic plexus of the Phaneroglossa is formed in _Rana_ by the VIII. +
IX. + X. + XIth nerves, the tenth issuing between the sacral vertebra and
the coccyx. In _Bufo_ and _Hyla_ the plexus is composed of five nerves, the
seventh spinal sending a branch to it. Occasionally the twelfth nerve
contributes a small branch to the posterior portion of the plexus. This and
the eleventh nerve leave the coccyx by separate holes, thereby indicating
its composition. The rest of the spinal cord gives off no more recognisable
nerves, owing to its reduction during the later stages of metamorphosis;
its terminal filament passes out of the posterior end of the coccygeal
canal.
Concerning the CRANIAL NERVES it is necessary to draw attention to one
point only. The last nerve which leaves the cranium of the Amphibia is the
vagus or tenth cranial nerve. There is consequently no eleventh, and no
twelfth or hypoglossal, pair of cranial nerves. Their homologues would be
the first and second spinal nerves, but the whole tongue of the Amphibia,
with its muscles, is supplied by the glossopharyngeal, or ninth cranial
pair, and is morphologically not homologous with the tongue of the Amniota.
{40}RESPIRATORY ORGANS
A very important and characteristic feature of the Amphibia is the
development of two sets of respiratory organs: Gills and Lungs. It is as
well to give definitions of these organs. _Lungs_ are hollow evaginations
from the ventral wall of the pharynx, and their thin, vascularised walls
enable the blood to exchange, by osmosis, carbon dioxide for oxygen from
the air which enters the lungs by the mouth or the nostrils, and the
windpipe. The latter is unpaired, the lungs themselves are paired. _Gills_
are highly vascularised, more or less ramified excrescences, covered by a
thin epithelium of ecto- or endo-dermal origin, which permits of the
exchange of carbon dioxide for oxygen from the air which is suspended in
the surrounding water. It is obvious that this definition applies to all
sorts of well-vascularised organs whose thin surface comes into contact
with the water. Various recesses of the pharyngeal cavity, the dorsal and
ventral folds of the tail-fin, nay, even any part of the skin of the body
can, and does occasionally, assume additional respiratory functions. The
proper definition of gills, in Vertebrates, requires, therefore, the
restriction that they must be developed upon and carried by visceral
arches.
The general statement that the Amphibia breathe by lungs, and, at least
during some stage of their life, also by gills, requires various
restrictions. As a rule the majority of Amphibia first develop gills, later
on also lungs, whereupon, during the metamorphosis, the gills are gradually
suppressed, so that the perfect animal breathes by lungs only (see p. 61).
But a number of Urodela retain their gills throughout life, although the
lungs are also functional. These are the Perennibranchiata, not a natural
group, but a heterogenous assembly, Proteidae and Sirenidae. Some species
of _Amblystoma_ remain individually Perennibranchiate (cf. Axolotl,
p. 112). On the other hand, in some Anura the gills are almost or entirely
suppressed, or restricted to the embryonic period only. Lastly, a
considerable number of Salamandridae have lost their lungs; they breathe by
gills until their metamorphosis, but have in the adult state to resort to
respiration by the skin (cf. p. 46).
The general plan of the development of the branchial respiratory apparatus
is as follows:–The six visceral arches, {41}namely, the mandibular, the
hyoidean, and the four branchial arches, correspond, long before they are
cartilaginous, with four main arterial arches of the truncus arteriosus.
The first, the arteria hyo-mandibularis, belongs to the hyoidean and
mandibular segments, the second to the first branchial, the third to the
second branchial, while the fourth soon splits in two for the third and
fourth or last branchial arch. On the dorsal side these branchial arterial
arches combine to form the radix of the dorsal aorta. These arches,
especially the three branchials, appear in newts, less clearly in frogs, as
transverse ridges on the sides of the future neck. Between the arches the
pharynx gradually bulges out in the shape of five lateral gill-pouches; the
first between the mandibular and the hyoidean arch, the second between the
hyoidean and the first branchial arch, etc. These pouches soon break
through to the outside and become gill-clefts, except the first pouch in
Urodela. _Before_ the breaking through of the clefts there appears upon the
outside of the middle of the rim of each arch a little knob, which soon
ramifies and forms an external gill. The knob owes its origin to the
development of a blood-vessel which buds from the arterial arch, ramifies
and breaks up into capillaries, and returns a little further dorsalwards
into the arch. A secondary loop to the outside of the primary arterial arch
is thus formed; and whilst this outer loop sprouts out further, driving
before it the likewise proliferating skin, and thus producing the gill, the
middle portion of the primary arch remains in the Urodela as a short cut,
but in the Anura it partly obliterates, and henceforth acts as the internal
_efferent_ vessel of the gill. When, during metamorphosis, the gills
disappear, their intrinsic _afferent_ and _efferent_ vessels vanish
likewise, and the short cut completes the circuit. In order to do this they
have, in the Anura, to form new connections with the trunks of the afferent
vessels.
The arterial arches themselves are modified as follows:–The first pair
become the carotids, the second form the right and left aortic arches,
while the third and fourth unite and are transformed into the pulmonary
arteries and "ductus Botalli," the last arterial arch having previously
sent a branch into the developing lungs. In the Anura the third arch
obliterates.
The gills and clefts present various modifications. The Urodela possess
three pairs of gills, one each upon the dorsal {42}half of the three
branchial arches, just near the upper corners of the clefts; and the skin
of the body is continued upon the stem of each gill, pigmented like the
rest of the surface of the body. Such a gill is more or less like a blade,
standing vertically, and is composed of a stem of connective tissue, thick
at the base, and, as a rule, carrying two series of fine lamellae, which,
however, do not form two opposite series, but hang downwards, being, so to
speak, folded down, so that the upper surface of the stem is bare, and
carries the lamellae on its under side. In the Axolotl some of these
lamellae are further subdivided. In _Necturus_ they are enormously
increased in numbers, but are rather short, and they stand no longer in two
rows, but are crowded into one. Those of _Proteus_ form two rows of
dendritic filaments; those of _Siren_ are likewise much ramified.
The larvae of the Urodela have four clefts. In the adult _Siren_ these are
reduced to three, the first, namely, that between the hyoid and the first
branchial arch, being closed up. In _Necturus_, _Proteus_, and
_Typhlomolge_ the clefts are further reduced to two, owing to the closing
up of the first and last, only those between the first, second, and third
arches remaining. _Amphiuma_, and usually _Cryptobranchus alleghaniensis_,
possess only one pair of clefts, while in _C. japonicus_ and in the
Salamandridae all the clefts are abolished.
The gills of the Urodela are always uncovered, although a short operculum
is formed from the posterior margin of the hyoidean arch; the halves of
this fold meet below the throat, and persist in various terrestrial and
aquatic species as the "gular fold." It reaches its greatest size just
before metamorphosis, but scarcely ever produces a proper outer
gill-chamber, and it does not cover the gills owing to their rather
pronounced dorsal position. It is perhaps best developed in _Typhlomolge_,
and even there its dorsal portion is continued upon the first of the three
broad vertical and short-fringed blades which form the gills.
A description of the gills of the Apoda will be found in the systematic
part.
In the Anura the gills are complicated, owing to the development of the
so-called internal gills. First appear, exactly in the same way as in the
Urodela, the external gills, one upon each of the first three branchial
arches. In the larva of _Rana esculenta_, 5 mm. in length, a little
protuberance appears upon the first, {43}and then upon the second arch. In
the 6 mm. larva the first gill shows four knobs, the second two, the third
one knob. They are always delicate and thin, although sometimes pigmented,
long, and much-ramified structures. The first pair is always the largest;
well developed and persisting a long time in _Rana temporaria_; smaller in
_R. esculenta_ and _Bufo vulgaris_; very short, scarcely forked, in _B.
viridis_ and _Hyla arborea_. They are relatively largest in _Alytes_, while
still in the egg. Numerous descriptions of these gills will be found in the
systematic part.
Great changes take place about the time when the fourth or last branchial
arch and the pulmonary arteries are developed. This occurs in _R.
esculenta_ when the larva is about 9 mm. long. The sprouting of the gills
extends gradually downwards along the arches upon their ventral halves, and
these new gill-filaments or loops transform themselves into numerous
dendritic bundles, resting in several thickset rows upon the hinder margin
of the first to the third arch, one row only on the fourth arch, which
carries no external gill. These "internal gills" look like red bolsters or
thick and short-tasselled bunches. Whilst they are developing the dorsal,
older gills become arrested in their growth and disappear, and at the same
time a right and left opercular fold grows out from the head and covers
these new gills, shutting them up in an outer branchial chamber, just like
that of Teleostei and other Tectobranch fishes. This is the reason why
these new gills have been called internal, and the mistaken notion has
sprung up that they are comparable with the true internal gills of fishes.
In reality Amphibia have only external gills. They are always covered by
ectoderm, are restricted to the outside of the branchial arches, and are
developed before the formation of the clefts. These gills are in many cases
directly continuous with the more dorsally and more superficially placed
earlier external gills; but although nearly every one who has studied their
development has observed this agreement, the old error still prevails. They
are morphologically as little internal as the true internal gills of
Elasmobranch embryos are external gills, because these have become so
elongated that they protrude out of the gill-clefts.
The fact that the Amphibia possess only external gills throws important
light upon their phylogeny. Not only do the Apoda, Urodela, and Anura agree
much more with each other than {44}would be the case if the Anura possessed
both internal and external gills, but the Amphibia reveal themselves also
in this point as connected with the Crossopterygii and the Dipnoi, some of
which fishes also possess external gills. It is of course quite possible
that the Amphibia have developed these organs independently, but we
understand now that the latter are accessory, and not the primitive
respiratory organs; they are developed in adaptation to embryonic
conditions and to prolonged larval, occasionally perennibranchiate, aquatic
life (cf. the chapter on Neoteny, p. 63).
There is no valid reason for supposing that the Stegocephali had true
internal gills. We know their branchial skeleton, and we can discern even
gill-rakers on the arches. Such gill-rakers occur also, although but feebly
developed, in Urodela. The whole branchial framework of the Urodela and
Apoda undergoes simple reductions during metamorphosis (see p. 86), but in
the Anura these arches are in early tadpole life transformed into a most
complicated basket-work which acts as a straining apparatus or filter, to
prevent any particle of food or other foreign matter from finding its way
into the delicate gills, the current of water passing from the mouth
through the filter, past the gills and out of the clefts. During
metamorphosis this whole elaborate apparatus is again transformed, almost
beyond recognition, into the hyoidean apparatus for the support of the
generally very movable and much-specialised tongue. The fact that the hyoid
apparatus of the Aglossa, especially that of _Xenopus_, is constructed upon
the same lines, is a strong indication that these creatures have arrived at
their tongueless condition through the loss of this organ, and this is
intelligible in correlation with their absolutely aquatic life.
The opercular folds assume great dimensions in all tadpoles. They cover the
whole gill-region, thereby producing on either side an outer gill-chamber.
The posterior margins of the folds gradually become continuous with the
rest of the surface of the body. Each gill-chamber opens at first by one
lateral canal, usually called the spiracle. This condition prevails in the
tadpoles of the Aglossa. In the Discoglossidae the two canals gradually
converge and combine into one median opening on the middle of the belly. In
all the other Anura the right opening becomes closed, or rather its canal
passes over to and joins that of the {45}left side, both opening by one
short tube laterally on the left side, at a variable distance between the
eye and the vent. Hence the elegant terms of Amphi-, Medio-, and
Laevo-gyrinidae (γυρῖνος being the Greek for tadpole).
The external gills lead to a further consideration. _Protopterus_ possesses
a vestigial external gill on the shoulder-girdle. _Lepidosiren_ has them on
the gill-arches, resembling piscine internal gills, and _Polypterus_ has a
large biserially fringed external gill (in some cases not disappearing
until the fish is adult), which starts from the mandibular arch, at the
level of the spiracle or first visceral cleft, and overlaps the operculum
externally. The axis of this peculiar organ is possibly based upon the
homologues of the spiracular cartilages, which themselves are the
branchiostegal rays of the dorsal half of the quadrato-mandibular arch. The
branchiostegal rays of the hyoidean arch, at least their material, have
given rise to the elaborate opercular apparatus; and, in conformity
herewith, the hyomandibular itself is not known to carry a gill. Quite
possibly the large external gill of _Polypterus_ is not serially homologous
with other external gills–it may not be a true gill at all, it has perhaps
quite a different function–but it seems to throw light upon a mysterious
pair of organs which are common in larval and young Urodela, in the larval
Aglossa and in the Apoda. These are the "balancers."
In _Triton taeniatus_, before hatching, there appears a little protuberance
behind and below the eye; it rests upon the angle of the mandibular arch,
and is separated from the first transverse, externally visible ridge of the
first branchial arch by the beginnings of the hyoidean arch. A few days
later the arteria hyomandibularis sends a vessel into this knob, forms a
vascular coil, and leaves it as a vein which, instead of returning into the
arterial arch, passes into the veins of the body. Its epithelium is not
covered with flat, but with cubical cells; and sensory cells have not been
found in it. These organs attain some size, and are shaped like rods, with
thickened ends; they are movable, and are used by the larvae as
"balancers," keeping the head from sinking into the slime at the bottom.
But they may have other functions besides, and it is not unlikely that they
develop into sensory organs like feelers. They occur in many Salamandridae,
and are not reduced until, or even after, the metamorphosis, and during
this time they shift their place with relation to the eye and the mouth.
{46}The same kind of organs occur in _Amblystoma_.[18] They appear,
previous to the breaking open of the gill-clefts, as protrusions of
epiblast, long before any of the external gills on the branchial arches.
When the clefts have broken open, the quadrate sends out laterally a tiny
crescent-shaped process a little above the jaw-joint, and this process
extends to the base of the balancer, but not into it, and a bundle of
muscle-cells grows into the balancer. It is easy to recognise the same
organ in the extremely long thread-like structures of the larva of
_Xenopus_. In the Apoda they are likewise present, but are retained
permanently as highly specialised, probably tentacular organs (cf. p. 86,
Apoda).
One of the most unexpected features is the SUPPRESSION OF THE LUNGS in
various kinds of Salamandridae. The lungs are either reduced to useless
vestiges or they are quite absent. This occurs in aquatic and terrestrial,
American and European forms, and it is noteworthy that the reduction of the
lungs does not apply to all the species of the various genera, nor is it
restricted to one sub-family.
The following list is due to the researches of H. H. Wilder,[19] L.
Camerano,[20] E. Lönnberg,[21] and G. S. Hopkins[22]:–All the
Desmognathinae and Plethodontinae; Amblystomatinae, _Amblystoma opacum_;
Salamandrinae, _Salamandrina perspicillata_. In _Triton_ and other
Salamandrinae the length of the lungs varies; in some they extend more, in
others less, than half way down the distance between head and pelvis.
Hopkins remarks: "Two questions are naturally suggested by this apparently
aberrant condition of the respiratory organs. First, what structures or
organs have taken on the function of the lungs and branchiae; and secondly,
is there any modification in the form or structure of the heart which in
any way may be correlated with the above-mentioned peculiarities of the
lungless forms?" Wilder concluded that respiration was probably carried on
by the skin, and perhaps, to some extent, by the mucosa of the intestine.
Camerano thinks that, at least in the European forms, respiration is
effected by the bucco-pharyngeal cavity, and that the skin affords no
efficient aid. The left auricle in the lungless forms is much {47}smaller
in comparison than the right, and there is no pulmonary vein. The auricular
septum has a large aperture, the communication between the auricles being
larger than even in _Necturus_ (which breathes essentially by gills). The
sinus venosus, instead of opening into the right auricle only, opens more
freely into the left than into the right, and the latter communicates more
directly with the ventricle than the left, instead of about equally. In
short, the heart of these creatures appears almost bilocular, instead of
being trilocular, at least functionally.
The lungs of the Urodela are always simple, extremely thin-walled bags.
They are highly developed in the Anura, the walls being modified into
numerous air-cells, whereby the respiratory surface is considerably
increased. The lungs are filled with air by the pumping motion of the
throat while the mouth is closed, the nostrils being provided with muscular
valves. A muscular apparatus assists the filling of the lungs in the
Anura.[23]
[Illustration: FIG. 6.–Internal view of the mouth of A, _Rana esculenta_,
B, _Bufo calamita_ (cf. Fig. 52, p. 269). _Ch_, Choana, or inner nasal
opening; _E_, opening of the Eustachian tube; _S_, slit leading into the
vocal sac; _T_, tongue; _Vo_, patches of teeth on the vomers.]
Most, if not all, Anura and some Urodela have a VOICE produced by the
larynx, which, especially in the Anura, is provided with a complicated
cartilaginous and muscular apparatus and with vocal cords. The voice of the
Urodela is at the best a feeble squeak. The females of the Anura are either
mute or they produce a mere grunt, but that of many males is very loud,
and, moreover, in many species it is intensified by _vocal sacs_ which act
as resonators. These sacs are diverticula of the lining of the
mouth-cavity, and bulge out the outer skin and the muscles, chiefly the
mylo-hyoid, of the throat. The nostrils and the mouth are firmly closed
during the croaking. "The sacs are called internal when they are covered by
the unmodified gular integument, however much this may be distended;
external when their membrane projects through slits at {48}the sides of the
throat, as in _Rana esculenta_ (Fig. 52, p. 269), or when the skin is
thinned and converted into a bladder-like pouch, as in _Hyla arborea_."[24]
These sacs exhibit many modifications. They may be unpaired and median, and
open by two slits into the mouth, on either side below the tongue; in
_Bufo_ one of the slits or openings, either the right or the left, is
obliterated. They may be paired and symmetrical, and open one on each side
of the head, below and near the posterior angle of the jaws. These
modifications differ in closely allied species. They reach their greatest
complication in _Rhinoderma_ and in some of the Cystignathidae by extending
far back beneath the skin into the wide lymphatic spaces. In _Rhinoderma_
they are put to the unique use of nurseries for the young (see p. 228).
_Leptodactylus typhonius_ has a very distinct pair of outer vocal sacs and
a well-marked unpaired sac which extends into the belly and communicates
with each outer sac. Several species of _Paludicola_, e.g. _P.
fuscomaculata_ and _P. signifera_, have a similar arrangement, in addition
to an unpaired gular sac which can be inflated independently of the rest
(see Fig. 45, p. 220).
URINO-GENITAL ORGANS
The kidneys and the male generative glands are still intimately connected
with each other. The general plan is as follows:–
The kidneys consist of a large number of glomeruli, produced by the coiled
segmental tubes, each of which is composed of a nephrostome or funnel
opening into the body-cavity, a Malpighian body and an efferent canal. The
latter combine to form the segmental duct which opens into the cloaca. The
testes, composed of a large number of sperm-producing glands, are drained
by transverse canals which combine into a longitudinal canal, and this
again sends off numerous efferent canals which open into the efferent
canals of the kidney, so that the segmental duct (Leydig's duct of many
authors) conveys both sperma and urine.
{49}[Illustration: FIG. 7.–Diagrammatic representation of modifications of
the urino-genital ducts. 1, 2, Male and female Newt; 3, a tubule of the
kidney; 4, male _Rana_; 5, male _Bufo_; 6, male _Bombinator_; 7, male
_Discoglossus_; 8, male _Alytes_. _a_, Artery entering, and producing a
coil in, the Malpighian body, _M_; _B_, Bidder's organ; _ef.s.c_, efferent
segmental canal; _F.B_, fat-body; _gl_, glomerulus; _K_, kidney; _l.c.c_,
longitudinal collecting canal; _M_, Malpighian body; _Md_, Müllerian duct;
_N_, nephrostome; _O_, ovary; _Ov_, oviduct; _s.d_, segmental duct; _T_,
testis; _Ur_, ureter; _V.d_, vas deferens; _V.s_, vesicula seminalis.]
{50}In the female the network of transverse and longitudinal canals, which
originally connect the generative glands with the kidney's efferent canals,
is deduced in so far as the connection is interrupted and the vestiges of
the transverse canals are no longer functional. The eggs fall into the
body-cavity and are caught up by the ostium or inner abdominal opening of a
special duct, the oviduct (Müllerian duct of many authors). Vestiges, more
or less complete, of these oviducts persist in the males of most Amphibia.
This general scheme presents some modifications in the various groups of
Amphibia.
The Apoda retain the most primitive conditions. The kidneys are still long
and narrow, and the glomeruli are, at least in the anterior part of the
organ, still strictly segmental, agreeing in number and position, each with
a vertebral segment; later, the number of the glomeruli is greatly
increased, and the former agreement becomes quite disturbed. The generative
glands still retain their segmental arrangement, but they are restricted to
a much shorter region than the kidneys. In the male Apoda a considerable
portion of the cloaca can be everted by special muscles, and acts as an
intromittent organ. Both sexes possess a ventral urinary bladder.