Caecilian Morphology
Caecilians are adapted primarily for burrowing and exhibit many of the same
morphological characteristics shown by other fossorial ectotherms such as
snakes, lizards, amphisbaenians, and salamanders. The Typhlonectids also show
secondary adaptations for an aquatic or semi-aquatic existence. This page will
cover some of the basics of caecilian morphology.
 Body Form
Elongation of the body and the reduction or loss of limbs is a common adaptation
in all burrowing tetrapods and caecilians are extreme examples of this. The result
is a worm-like or snake-like appearance. Some aquatic species
(Typhlonectes, Atretochoana, and Potomotyphlus) have a small,
raised "fin" along the posterior portion of the body to aid in swimming
and the result is an eel-like appearance. There is no trace of limbs or pelvic or pectoral
girdles left in caecilians. By comparison, some caecilian-like salamanders still have at
least two legs and some snakes and legless lizards still have remnants of legs under the skin.
The fossil caecilian, Eocaecilia, had reduced legs.
The body is arranged in rings, called annuli (Figure 1). These annuli give caecilians their
earthworm-like appearance. Additionally, the annuli may be further divided into secondary and
tertiary annuli. Having secondary or tertiary annuli is considered to be a more primitive
trait among caecilian families.
 Although it can be difficult to conceptualize unless you have examined a caecilian in
person, the tail makes up only a very small part of the body when it exists at all. In
the majority of species, the body simply ends abruptly in a blunt terminal "shield"
or "terminus" (Figure 2). Only the more primitive families of caecilians still retain a tail.
Body size varies markedly within the Gymnophiona, but is certainly constrained by a fossorial lifestyle.
The smallest known caecilians are probably Idiocranium russeli from Cameroon with a largest recorded
length of 14.4cm, though females as small as 9cm have been found brooding eggs. Individuals as small as
5.1cm have also been found with no trace of larval characteristics. On the Seychelles Archipelago,
Grandisonia brevis (11.2cm) and G. dimunitiva (9.5cm) often are shorter in length
than Idiocranium, but considerably thicker and heavier in body form.
The longest caecilian known is Caecilia thompsoni at 151.5cm but other species such
as Caecilia tentaculata and Caecilia nigricans, each reaching lengths of 100cm
or more, are much stouter and heavier.
The Skull
Caecilian skulls are strongly built and heavily ossified. The head acts as a battering
ram as caecilians push and prod their way through soil. As a result, the skull is wedge-
like and bones are heavily fused. Aside from sensory openings, the skulls of most caecilians
are completely roofed with thick bone, a condition called stegokrotaphy (Figure 3). Other
more primitive species still retain a partially open temporal region, a condition known as
zygokrotaphy (Figure 4). A reduction of the number of bones in the skull is considered an
advanced trait in caecilian families.
The Eyes
 All caecilians have eyes, but they may be so reduced and covered by skin or bone that you
may not be able to see them. Leading a fossorial life, the eyes have degenerated to various
degrees in different species. Some species, such as Ichthyophis, have superficially
placed eyes while other species such as Herpele (Figure 5) and Gegeneophis have eyes beneath the bones of the skull and have even had their eye sockets replaced by bone
(Figure 6).
 Comparative morphological studies suggest that there is a trend towards increased covering of the
eye by skin or bone in conjunction with a loss of the lens and modifications of the retina. However,
the retina and optic nerve remain intact and so it is likely that most caecilian eyes are still
capable of photoreception.
Some caecilian keepers report that their animals do not show any reaction to lighting. Others
do seem to notice an ability to detect light and report that their caecilians will often hide
or jerk away when a bright light is suddenly turned on in their enclosure or is pointed at
their eyes. They do not seem capable of detecting movement visually.
 The Tentacles
A small sensory tentacle is always present on either side of the head between the eye
and the nostril (Figure 7) and the exact positioning of the tentacles is highly variable
among species. In most species, it can be protruded through an aperture in the skull
while in others it is not protrusible (Typhlonectids). The family Scolecomorphidae is
somewhat famous for having the tentacle in close association with the eye; when the
tentacle is protruded, the eye is carried along outside the cranium on the tip of the
tentacle.
The tentacle is a complex structure of many forms of tissues including nerves, muscles, ducts,
and glands and is thought to function in chemoreception.
The Mouth and Teeth
 In most caecilians, the mouth is not positioned directly at the end of the head
(terminal), but rather is positioned slightly below (subterminal). This arrangement is
also called "countersunk and is considered an adaptation for burrowing (Figure 8).
The most primitive caecilians still possess terminal mouths.
The upper and lower jaws each carry two rows of teeth, an inner row and an outer row. These teeth vary
in size and shape depending on species and tooth counts change with age. Captive Typhlonectes
have been known to mouth the hands and fingers of their keepers at feeding time, which is
quite harmless and painless.
 Nuchal Collars and Grooves
Immediately behind the head are two anatomical structures called nuchal collars, which are
differentiated from each other and the rest of the body by nuchal grooves (figure 9).
Somewhat similar in appearance to the clitellum of earthworms, the nuchal collars usually
span 3 to 4 vertebrae and are most noticeable when viewing the caecilian from the side or
below. The first nuchal groove marks the posterior border of the skull and the second marks
the division between the two nuchal collars. The third nuchal groove marks the boundary
between the second nuchal collar and the rest of the body. The nuchal collars may sometimes
be difficult to distinguish in some species due to additional dermal folds along the dorsal
surface.
The gills of larval caecilians often occur on the same plane as the second or third nuchal groove.
Internal Organs
Like snakes, amphisbaenians, salamanders, and legless lizards, the internal organs of caecilians
have become greatly elongated. One or more of the lungs may be reduced or absent entirely,
usually the left lung (Atretochoana eiselti lacks lungs entirely).
Jaw Muscles
All terrestrial vertebrates, except caecilians, have a single set of jaw-closing muscles.
Caecilians have two sets of muscles (mandibular adductors and interhyoideus muscles) for
this purpose and this is thought to be an adaptation for holding the jaws firmly closed while
burrowing.
The Skin
Like other amphibians, caecilians do possess poison glands in the skin although the potency
of such poisons is not yet well known. Toxic skin secretions have been observed for
Typhlonectes compressicauda and hemolytic and cardiotoxic properties have been
found in the skin secretions of Siphonops paulensis .
Unlike other extant amphibians, caecilians possess scales under the skin, which are composed of
collagenous fibers covered with mineralized nodules. These can be found in the folds and grooves
of the skin and usually increase in number posteriorly. Additionally, Caecilia sp. have a
second type of scale which is embedded into the subdermal connective tissue.
References:
Cogger, H.G., & R.G. Zweifel. 2003. Encyclopedia of Reptiles and
Amphibians: A Comprehensive Illustrated Guide by International Experts. Fog City
Press, San Francisco.
Measey, J.G., Gower, D.J., Oomen, V.O., & M. Wilkinson. 2004. A Subterranean
Generalist Predator: Diet of the Soil Dwelling Caecilian Gegeneophis ramaswamii
(Amphibia; Gymnophiona, Caecilidae) in Southern India. C.R. Biologies 327: 65-76.
Pillai, R.S. & M.S. Ravichandran. 1999. Gymnophiona of India: A Taxonomic Study.
Rec. Zoological Survey India, Occasional Paper No. 172: 1-126.
Pough, F.H., R.M. Andrews, J.E. Cadle, M.L. Crump, A.H. Savitzky, K.D. Wells. 1998.
Herpetology. Prentice Hall Press, New Jersey.
Taylor, Edward H. 1968. The Caecilians of the World: A Taxonomic Review. University
of Kansas Press, Lawrence.
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