Crocodile exoskeleton
Encyclopedia
The crocodile exoskeleton consists of the protective dermal
and epidermal components of the integumentary system
in animals of the order
Crocodilia
. It is a form of armour
.
of the alligator
consists of oblong horny scale
s, arranged in transverse rows; the long axes of the scales are parallel to that of the body. On the tail, except along the mid-dorsal
line, and on the ventral side of the trunk and head these scales are very regular in outline and arrangement; on the sides of the head and trunk and on the legs they are much smaller and less regularly arranged, while along the mid-dorsal line of the tail, especially in its posterior half, they are elevated into tall keels that give the tail a large surface area for swimming. The first three digits of both manus (fore foot) and pes (hind foot) are armed with horny claws, which also belong to the epidermal part of the exoskeleton.
The dermal exoskeleton
consists of bony
scute
s that underlie the epidermal scales of the dorsal surface of the trunk and anterior part of the tail. The overlying scales, except in very young animals, are always rubbed off, so that the bony scales are exposed. The ventral or inner surface of the scutes is flat, while the outer surface is strongly keeled and in old animals is often rough and pitted. The plates are nearly square in outline and are closely joined together in most places.
The scutes are grouped in two fairly distinct areas known as the nuchal and the dorsal shields. The former lies just back of the head, in the region of the fore legs, and consists of four larger and a number of smaller plates. The latter, or dorsal shield, extends over the back in fairly regular longitudinal rows and quite regular transverse rows. At the widest part of the trunk there are six or eight of these scutes in one transverse row. They become smaller towards the tail.
The teeth are exoskeletal structures, partly of ectodermal, partly of dermal origin. They are conical in shape, without roots, and are replaced when lost. They will be described in connection with the skull.
Musk
glands, said by Gadow to be present in all Crocodilia, are found in both sexes and are derivations of the skin. One pair, each of which may be as large as a walnut, is found on the lower side of the head, one on the inside of each half of the mandible. The other pair is inside of the lips of the cloaca
.
, young, or half-grown Crocodilia contains the rete Malpighii: a single layer of short, cylindrical cells. Over the rete are somewhat flattened, disk-shaped cells formed by transverse division of the underlying rete cells. On the outside lies the epitrichial layer, a mosaic of polygonal cells each with an oval nucleus near its middle. Between the epitrichial cells are small oval holes, not unlike the stomata in the epidermis of plant tissues. Bronn thinks these are not artifacts, but he does not suggest any explanation of their occurrence.
On the short, cylindrical rete Malpighii cells are flattened cells that gradually become very flat and lose their nuclei as they pass over into the horny layer.
The stratum corneum consists of strongly flattened cells in which the nuclei can no
longer be clearly seen, though their location can usually be determined by the groups of pigment granules. On the cells of the more superficial layers of the stratum corneum are straight, dark lines, perhaps ridges caused by pressure of the over- or underlying polygonal cells. The individual cells of the horny layer are usually easily isolated in the belly and neck regions where they never become very thick; but in the back the cells in this layer are very numerous and fuse with each other to form the bony plates; here the rete is the only clearly differentiated layer. Whether prickle cells are present in the epidermis of the crocodile Bronn is not certain, though he thinks they probably are.
Rathke pointed out that on the surface of certain folds of the integument, especially in the region of the jaws, are found in all Crocodilia certain small, scattered, wart-like elevations, around each of which is customarily a narrow, shallow, circular groove; they usually have a dark brown but sometimes a gray or even white color. Microscopic examination shows these warts to be of epidermal origin, consisting of bright, round cells that are closely united, without visible intercellular substance. Treatment with potassium hydroxide and then with water will show sometimes, though not always, fine granular nuclei in the cells. In probably all members of the genus Crocodilus, at least is found, on the thick
swelling on the right and on the left side of the neck and trunk, a small, flat pit which has the appearance of the opening of an integumental gland. The pits are present also in the scales of the throat, under the side of the neck, sides of the body, lateral and ventral surfaces of the anterior half of the tail, and the legs. They are near the hinder border of the scales. Only occasionally are two pits found in one scale. These pits are found in the gavials but are absent in some, probably all, alligators. A small knob projects from the center of some of the pits. These pits are not openings of glands but have about the same structure as the pits seen in the head.
The integumental bones in the Crocodilia originate in the connective tissue of the cutis
. Investigations in young animals show that these bones usually take their origin in the under and middle layers of the cutis and generally work towards the periphery.
Dermis
The dermis is a layer of skin between the epidermis and subcutaneous tissues, and is composed of two layers, the papillary and reticular dermis...
and epidermal components of the integumentary system
Integumentary system
The integumentary system is the organ system that protects the body from damage, comprising the skin and its appendages...
in animals of the order
Order (biology)
In scientific classification used in biology, the order is# a taxonomic rank used in the classification of organisms. Other well-known ranks are life, domain, kingdom, phylum, class, family, genus, and species, with order fitting in between class and family...
Crocodilia
Crocodilia
Crocodilia is an order of large reptiles that appeared about 84 million years ago in the late Cretaceous Period . They are the closest living relatives of birds, as the two groups are the only known survivors of the Archosauria...
. It is a form of armour
Armour (zoology)
Armour in animals is external or superficial protection against attack by predators, formed as part of the body , usually through the hardening of body tissues, outgrowths or secretions. It has therefore mostly developed in 'prey' species...
.
Structure and anatomy
The epidermal exoskeletonExoskeleton
An exoskeleton is the external skeleton that supports and protects an animal's body, in contrast to the internal skeleton of, for example, a human. In popular usage, some of the larger kinds of exoskeletons are known as "shells". Examples of exoskeleton animals include insects such as grasshoppers...
of the alligator
Alligator
An alligator is a crocodilian in the genus Alligator of the family Alligatoridae. There are two extant alligator species: the American alligator and the Chinese alligator ....
consists of oblong horny scale
Scale (zoology)
In most biological nomenclature, a scale is a small rigid plate that grows out of an animal's skin to provide protection. In lepidopteran species, scales are plates on the surface of the insect wing, and provide coloration...
s, arranged in transverse rows; the long axes of the scales are parallel to that of the body. On the tail, except along the mid-dorsal
Dorsum (biology)
In anatomy, the dorsum is the upper side of animals that typically run, fly, or swim in a horizontal position, and the back side of animals that walk upright. In vertebrates the dorsum contains the backbone. The term dorsal refers to anatomical structures that are either situated toward or grow...
line, and on the ventral side of the trunk and head these scales are very regular in outline and arrangement; on the sides of the head and trunk and on the legs they are much smaller and less regularly arranged, while along the mid-dorsal line of the tail, especially in its posterior half, they are elevated into tall keels that give the tail a large surface area for swimming. The first three digits of both manus (fore foot) and pes (hind foot) are armed with horny claws, which also belong to the epidermal part of the exoskeleton.
The dermal exoskeleton
Exoskeleton
An exoskeleton is the external skeleton that supports and protects an animal's body, in contrast to the internal skeleton of, for example, a human. In popular usage, some of the larger kinds of exoskeletons are known as "shells". Examples of exoskeleton animals include insects such as grasshoppers...
consists of bony
Bone
Bones are rigid organs that constitute part of the endoskeleton of vertebrates. They support, and protect the various organs of the body, produce red and white blood cells and store minerals. Bone tissue is a type of dense connective tissue...
scute
Scute
A scute or scutum is a bony external plate or scale, as on the shell of a turtle, the skin of crocodilians, the feet of some birds or the anterior portion of the mesonotum in insects.-Properties:...
s that underlie the epidermal scales of the dorsal surface of the trunk and anterior part of the tail. The overlying scales, except in very young animals, are always rubbed off, so that the bony scales are exposed. The ventral or inner surface of the scutes is flat, while the outer surface is strongly keeled and in old animals is often rough and pitted. The plates are nearly square in outline and are closely joined together in most places.
The scutes are grouped in two fairly distinct areas known as the nuchal and the dorsal shields. The former lies just back of the head, in the region of the fore legs, and consists of four larger and a number of smaller plates. The latter, or dorsal shield, extends over the back in fairly regular longitudinal rows and quite regular transverse rows. At the widest part of the trunk there are six or eight of these scutes in one transverse row. They become smaller towards the tail.
The teeth are exoskeletal structures, partly of ectodermal, partly of dermal origin. They are conical in shape, without roots, and are replaced when lost. They will be described in connection with the skull.
Musk
Musk
Musk is a class of aromatic substances commonly used as base notes in perfumery. They include glandular secretions from animals such as the musk deer, numerous plants emitting similar fragrances, and artificial substances with similar odors. Musk was a name originally given to a substance with a...
glands, said by Gadow to be present in all Crocodilia, are found in both sexes and are derivations of the skin. One pair, each of which may be as large as a walnut, is found on the lower side of the head, one on the inside of each half of the mandible. The other pair is inside of the lips of the cloaca
Cloaca
In zoological anatomy, a cloaca is the posterior opening that serves as the only such opening for the intestinal, reproductive, and urinary tracts of certain animal species...
.
Histology of the integument
The epidermis of an embryoEmbryo
An embryo is a multicellular diploid eukaryote in its earliest stage of development, from the time of first cell division until birth, hatching, or germination...
, young, or half-grown Crocodilia contains the rete Malpighii: a single layer of short, cylindrical cells. Over the rete are somewhat flattened, disk-shaped cells formed by transverse division of the underlying rete cells. On the outside lies the epitrichial layer, a mosaic of polygonal cells each with an oval nucleus near its middle. Between the epitrichial cells are small oval holes, not unlike the stomata in the epidermis of plant tissues. Bronn thinks these are not artifacts, but he does not suggest any explanation of their occurrence.
On the short, cylindrical rete Malpighii cells are flattened cells that gradually become very flat and lose their nuclei as they pass over into the horny layer.
The stratum corneum consists of strongly flattened cells in which the nuclei can no
longer be clearly seen, though their location can usually be determined by the groups of pigment granules. On the cells of the more superficial layers of the stratum corneum are straight, dark lines, perhaps ridges caused by pressure of the over- or underlying polygonal cells. The individual cells of the horny layer are usually easily isolated in the belly and neck regions where they never become very thick; but in the back the cells in this layer are very numerous and fuse with each other to form the bony plates; here the rete is the only clearly differentiated layer. Whether prickle cells are present in the epidermis of the crocodile Bronn is not certain, though he thinks they probably are.
Rathke pointed out that on the surface of certain folds of the integument, especially in the region of the jaws, are found in all Crocodilia certain small, scattered, wart-like elevations, around each of which is customarily a narrow, shallow, circular groove; they usually have a dark brown but sometimes a gray or even white color. Microscopic examination shows these warts to be of epidermal origin, consisting of bright, round cells that are closely united, without visible intercellular substance. Treatment with potassium hydroxide and then with water will show sometimes, though not always, fine granular nuclei in the cells. In probably all members of the genus Crocodilus, at least is found, on the thick
swelling on the right and on the left side of the neck and trunk, a small, flat pit which has the appearance of the opening of an integumental gland. The pits are present also in the scales of the throat, under the side of the neck, sides of the body, lateral and ventral surfaces of the anterior half of the tail, and the legs. They are near the hinder border of the scales. Only occasionally are two pits found in one scale. These pits are found in the gavials but are absent in some, probably all, alligators. A small knob projects from the center of some of the pits. These pits are not openings of glands but have about the same structure as the pits seen in the head.
The integumental bones in the Crocodilia originate in the connective tissue of the cutis
Cutis
Cutis may refer to several unrelated biological structures:* Cutis , the outermost layers of skin* Cutis , a type of pileipellis with a repent arrangement of hyphaeand to:* Cutiş, a village in Almaşu Commune, Sălaj County, Romania...
. Investigations in young animals show that these bones usually take their origin in the under and middle layers of the cutis and generally work towards the periphery.
Sources
- Bronn, H. G. Klassen und Ordnungen des Thier-Reichs, vol. 63, "Reptilien 2, Eidechsen und Wasserechsen". 1959.
- Gadow, H. Amphibia and reptiles. The Cambridge Natural History. MacMillan and Co. Ltd., London, 1923.
- Rathke, C. Untersuchen über die Entwickelung und den Körperbau der Crocodile. Braunschweig, 1866.
- Reese, Albert M. The Alligator and Its Allies (PDF). Originally published Knickerbocker Press, 1915; electronic reprint c2000 Arment Biological Press.