An endoskeleton (from Ancient Greek ἔνδον ( éndon), meaning "inside", and σκελετός ( skeletós), meaning "skeleton") is a structural frame (skeleton) — usually composed of mineralized tissue — on the inside of an animal, overlaid by .
and the closely related are the predominant animal clade with endoskeletons (made of mostly bone and sometimes cartilage, as well as glycoprotein and ), although such as also have evolved a form of "rebar" endoskeletons made of diffuse meshworks of calcite/silica structural elements called sponge spicule, and have a dermal calcite endoskeleton known as ossicles. Some coleoid ( and cuttlefish) have an internalized vestigial aragonite/calcite-chitin mollusc shell known as gladius or cuttlebone, which can serve as muscle attachments but the main function is often to maintain buoyancy rather than to give structural support, and their body shape is largely maintained by hydroskeleton.
Compared to the of many invertebrates, endoskeletons allow much larger overall body sizes for the same skeletal mass, as most soft tissues and organs are positioned outside the skeleton rather than within it, thus unrestricted by the volume and internal capacity of the skeleton itself. Being more centralized in structure also means more compact volume, making it easier for the circulatory system to perfusion and oxygenate, as well as higher tissue density against stress. The external nature of muscle attachments also allows thicker and more diverse muscle architectures, as well as more versatile range of motions.
Overview
A true endoskeleton is derived from
tissue. In three
phylum of animals,
Chordata (chordates),
(echinoderms) and
Porifera (sponges), endoskeletons of various complexity are found. An endoskeleton may function purely for structural support (as in the case of Porifera), but often also serves as an attachment site for
and a mechanism for transmitting muscular forces as in chordates and echinoderms, which provides a means of locomotion.
Compared to the exoskeleton structure in many (particularly ), the endoskeleton has several advantages:
-
The capacity for larger body sizes under the same skeletal mass, as the endoskeleton has a "flesh-over-bone" construct rather than a "flesh-in-bone" one as in exoskeletons. This means that the body's overall volume is not restricted by the endoskeleton itself, but by the weight of soft tissues that can be attached and supported by it, while the capacity of an exoskeleton's internal body cavity restricts how much organs and tissues can be supported. Because of skeletal rigidity, many invertebrates have to repeatedly moulting (ecdysis) during the juvenile stages of life to grow bigger.
-
Endoskeletons have a more concentrated layout due to its internalized nature, so a greater proportion of skeletal tissue can be recruited to handle . In contrast, exoskeletons are more "spread thin" over the exterior, meaning that when stress is applied to one area of the body, most of the remaining exoskeleton often just plays "dead weight". Increasing the skeletal strength of a local area often means having to increase the cuticle thickness and density of an entire part of the body, which increase the overall weight significantly, especially with larger body sizes.
-
Being internal means the skeletal tissue can be perfusion and maintained from both inside (via nutrient arteries of the bone marrow) and outside (via periosteal ). The tissue catchment volume that the circulatory system is required to cover is also smaller than that of exoskeletons, making it easier to maintain skeletal health.
-
Endoskeletons are typically cushioned from Injury by the overlying soft tissues, while exoskeletons are directly exposed to external insults.
-
Having other tissues attached outside the skeleton means that endoskeletons can have a more diverse muscular layouts as well as bigger physiological cross-sectional area, which translates to greater contractile strength and adaptability. Having external muscles also means the potential for greater as the muscle can attach further down from a joint (comparatively, exoskeletal muscles cannot attach farther than the internal diameter of the corresponding joint cavity), although the muscles (especially ) themselves can sometimes physically hinder the joint's range of motion.
Chordates
All
have a
notochord, a flexible
glycoprotein rod cross-wrapped by two
collagen-
elastin helices, which their
develop around as
. With the exception of the
subphylum Tunicata (whose members only retain the notochord during
stages and as
are either soft-bodied or, in the case of
, supported by a
cellulose exoskeleton known as a test), chordate bodies are developed along an
axial skeleton endoskeleton derived from the notochord. Like many macroscopically
motile bilaterian animals that need to be capable of sufficient locomotive
propulsion, chordates evolved specialized
over their endoskeletons, which have serialized
and parallel
bundled in
muscle fascicle to both generate greater
force and optimize contractile speed.
Cephalochordates
In the more basal subphylum
Cephalochordata (
), the endoskeleton solely consists of a single notochord. Alternating muscle contractions bend the notochord from side to side, which stores and releases
elastic energy like a spring, resulting in a body-caudal fin locomotion with better energy efficiency, although
extant taxon cephalochordates (only three
genera with 32
species from the family Branchiostomatidae) are
burrowing who mostly remain immobile in the substrate.
Vertebrates
Chordates in the
crown group subphylum
Vertebrata (i.e.
vertebrates, such as
fish,
,
,
and
), the endoskeleton is greatly expanded. During embryonic development, the notochord becomes
body segment replaced by a much tougher
vertebral column (i.e. the
spine) composed of stiffer structural elements called
. Notochord
vestigiality are transformed into intervertebral discs, which give some range of motion between the adjacent vertebrae, allowing the overall spinal column to flex and rotate. The vertebrate endoskeleton is made up of two types of mineralized tissues, i.e.
bone and
cartilage, with the
reinforced by
made of Type I collagen. Unlike the singular axial skeleton of cephalochordates, the vertebrate skeletal elements expand axially, ventrally and laterally to form the
cranium,
rib cage and appendicular skeleton, giving vertebrates a much more widened endoskeleton.
Vertebrates also have bulkier, more complexly organized striated muscles called inserted over both the axial and appendicular skeletons, which can transmit significant forces via dense connective tissue cords/bands called and aponeuroses. In terrestrial vertebrates (), both the axial and especially the appendicular endoskeleton (the latter of which evolution into limb skeletons) have become significantly strengthened to adapt for the added burden of gravity and locomotion on dry land, as their bodies' weight is not offset by buoyancy as in aquatic environments. In some vertebrate species, parts of the endoskeleton become specialized for animal flight (as ), balance (in arboreal species), communication (as animal language or fish fin/sail/crest display), hearing (mammalian ), digestion (particularly mastication) and prehensility (grasping, object manipulation and fine motor activities).
The combination of a more robust endoskeleton and a stronger, more versatile muscular system, supported by a heart-pumped closed circulatory system, a nervous system with faster saltatory conductions (in all ) and centralized neural control by an highly functional brain, have allowed the vertebrates to achieve much larger body sizes than while still maintaining responsive sensory perception and motor control. As a result, vertebrates have gradually dominated all trophic level ecological niche in both aquatic and terrestrial ecosystems since the Devonian (circa. 420-359 Mya).
Echinoderms
Echinoderms have a
skeleton in the
dermis, composed of
calcite-based plates known as ossicles, which form a porous structure known as
stereom.
In
, the ossicles are fused together into a test, while in the arms of
,
and
(sea lilies) they articulate to form flexible joints. The ossicles may bear external projections in the form of spines, granules or warts that are supported by a tough epidermis. Echinoderm skeletal elements are sometimes deployed in specialized ways such as the
chewing organ in sea urchins called "Aristotle's lantern", the supportive stalks of crinoids, and the structural "lime ring" of
.
Sponges
The poriferan "skeleton" consists of mesh-like network of microscopic
sponge spicule. The soft connective tissues of sponges are composed of gelatinous
mesohyl reinforced by fibrous
spongin, forming a composite matrix that has decent
tensile strength but severely lacks the
stiffness needed to resist deformation from
. The spicules act as structural elements that add much needed compressive and
that help maintain the sponge's shape (which is needed to ensure optimal
filter feeding), much like the aggregates and rebar stirrups within reinforced concrete. Sponges can have spicules made of calcium carbonate (
calcite or
aragonite) or more commonly
silica, which separate sponges into two main
, calcareous sponges (class
Calcarea) and
, the latter being the dominant extant clade with two classes
Demospongiae (
) and
Hexactinellida (
). There are however species (such as bath sponge and lake sponge) that have no or severely reduced spicules, which gives them an overall soft "spongy" structure.
Deep-sea demosponges from the family Cladorhizidae have evolved a unique carnivorous survival strategy, by having tiny grappling hook-like spicules () that extends outwards like to snag and trap passing-by aquatic animals such as small fish and . As sponges don't have dedicated , these predatory sponges rely on symbiotic organisms such as and to help digest the seized prey and release that can then be absorbed by the sponges' cells.
Coleoids
The
Coleoidea, a subclass of
cephalopod who
evolution an internalized
mollusc shell, do not have a true endoskeleton in the physiological sense. The internal shell has evolved into a
buoyancy organ called the gladius or
cuttlebone, which may provide muscle attachment but does
not support the cephalopod's body shape (which is maintained solely by a
hydroskeleton). Coleoids from the order
Octopus (octopuses) even have lost that internalized shell completely.
== Gallery ==
on display at Booth Museum of Natural History]]
, a cartilaginous fish]]
, an echinoderm]]
]]
See also
