A true endoskeleton is derived from
mesodermal tissue. In three
phyla of animals,
Chordata (chordates),
Echinodermata (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
muscles 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
invertebrates (particularly
panarthropods), 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
cavity restricts how much
organs and tissues can be supported. Because of skeletal rigidity, many invertebrates have to repeatedly
moult (
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
mechanical loads. 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
perfused and maintained from both inside (via
nutrient arteries of the
marrow) and outside (via
periosteal arterioles). 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
trauma 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
leverage 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
flexors) themselves can sometimes physically hinder the joint's
range of motion.
Chordates All
chordates have a
notochord, a flexible
glycoprotein rod cross-wrapped by two
collagen-
elastin helices, which their
body plans develop around as
embryos. With the exception of the
subphylum Tunicata (whose members only retain the notochord during
larval
stages and as
adults are either
soft-bodied or, in the case of
sea squirts, supported by a
cellulose exoskeleton known as a
test), chordate bodies are developed along an
axial endoskeleton derived from the notochord. Like many macroscopically
motile bilaterian animals that need to be capable of sufficient
locomotive propulsion, chordates evolved specialized
striated muscles over their endoskeletons, which have serialized
sarcomeres and parallel
myofibrils bundled in
fascicles to both generate greater
force and optimize
contractile speed.
Cephalochordates In the more
basal subphylum
Cephalochordata (
lancelets), 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 cephalochordates (only three
genera with 32
species from the family
Branchiostomatidae) are
burrowing filter feeders who mostly remain immobile in the
substrate.
Vertebrates Chordates in the
crown group subphylum
Vertebrata (i.e.
vertebrates, such as
fish,
amphibians,
reptiles,
birds and
mammals), the endoskeleton is greatly expanded. During
embryonic development, the notochord becomes
segmentally replaced by a much tougher
vertebral column (i.e. the
spine) composed of stiffer
structural elements called
vertebrae. Notochord
remnants 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
joints reinforced by
ligaments 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
skeletal muscles inserted over both the axial and appendicular skeletons, which can transmit significant forces via
dense connective tissue cords/bands called
tendons and
aponeuroses. In
terrestrial vertebrates (
tetrapods), both the axial and
especially the appendicular endoskeleton (the latter of which
evolved 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
flight (as
wings),
balance (in
arboreal species),
communication (as
vocalizations or
fin/
sail/
crest display),
hearing (
mammalian ossicles),
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
myelinated
nervous system with faster
saltatory conductions (in all
jawed vertebrates) and
centralized neural control by an highly functional
brain, have allowed the vertebrates to achieve much larger body sizes than
invertebrates while still maintaining responsive
sensory perception and
motor control. As a result, vertebrates have gradually dominated all
high-level niches in both
aquatic and
terrestrial ecosystems since the
Devonian (circa. 420-359
Mya).
Echinoderms Echinoderms have a
mesodermal skeleton in the
dermis, composed of
calcite-based plates known as
ossicles, which form a porous structure known as
stereom. In
sea urchins, the ossicles are fused together into a
test, while in the arms of
sea stars,
brittle stars and
crinoids (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
sea cucumbers.
Sponges The poriferan "skeleton" consists of mesh-like network of microscopic
spicules. 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
rigidity needed to resist
deformation from
ocean currents. The spicules act as
structural elements that add much needed
compressive and
shear strengths 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
clades,
calcareous sponges (
class Calcarea) and
siliceous sponges, the latter being the dominant extant clade with two classes
Demospongiae (
common sponges) and
Hexactinellida (
glass sponges). 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 (
microscleres) that extends outwards like
burs to snag and trap passing-by aquatic animals such as small fish and
crustaceans. As sponges don't have dedicated
digestive systems, these predatory sponges rely on
symbiotic organisms such as
scale worms and
microbes to help digest the seized prey and release
nutrients that can then be absorbed by the sponges' cells.
Coleoids The
Coleoidea, a
subclass of
cephalopod molluscs who
evolved an internalized
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 Octopoda (octopuses) even have lost that internalized shell completely. == Gallery ==