Endoskeleton

An endoskeleton (From Greek ἔνδον, éndon = "within", "inner" + σκελετός, skeletos = "skeleton") is a structural frame (skeleton) on the inside of an animal, overlaid by soft tissues and usually composed of mineralized tissue.^[1]^[2] Endoskeletons serve as structural support against gravity and mechanical loads, and provide anchoring attachment sites for skeletal muscles to transmit force and allow movements and locomotion.

Vertebrates and the closely related cephalochordates are the predominant animal clade with endoskeletons (made of mostly bone and sometimes cartilage), although invertebrates such as sponges also have evolved a form of "rebar" endoskeletons made of diffuse meshworks of calcite/silica structural elements called spicules, and echinoderms have a dermal calcite endoskeleton known as ossicles. Some coleoid cephalopods (squids and cuttlefish) have an internalized vestigial aragonite/calcite-chitin 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 exoskeletons 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 perfuse 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 mesodermal tissue. In three phyla of animals, Chordata, Echinodermata 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 "bone-over-flesh" one as an exoskeleton. 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 more skeletal tissue can be recruited to handle mechanical loads. In contrast, exoskeletons are more "spread thin" over the exterior, and increasing the skeletal strength often means having to increase the cuticle thickness and density of an entire part of the body, which increase the 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 arteries). The circulatory system is also required to cover a smaller catchment volume than that of exoskeletons.
Endoskeletons are typically cushioned from trauma by the overlying soft tissues, while exoskeletons are directly exposed external insults.
Having other tissues attached outside the skeleton means that endoskeletons can have a more diverse muscular layout as well as bigger physiological cross-sectional area, which translates to greater contractile strength. Having external muscles also means the potential for greater leverage as the muscle can attach further down (comparatively, exoskeletal muscles cannot attach farther than the internal diameter of the corresponding joint), although the muscles (especially flexors) themselves can sometimes physically hinder the joint's range of motion.

Chordates

Chordates, with the exception of the subphylum Tunicata (which are either soft-bodied or are supported by an exoskeleton known as a test), are developed along an axial endoskeleton. In the more basal subphylum Cephalochordata (lancelets), the endoskeleton consists of solely of an elastic glycoprotein-collagen rod called notochord, which stores energy like a spring and enable more energy-efficient swimming. In the crown group subphylum Vertebrata (vertebrates), the endoskeleton is greatly expanded, with the notochord replaced by a segmented vertebral column and the skeletal elements develop further to form the cranium, rib cage and appendicular skeleton. The vertebrate endoskeleton is made up of two types of mineralized tissues, i.e. bone and cartilage, reinforced by collagenous ligaments.

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.^[3]^[4] 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.^[5]

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 and siliceous 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.

Coleoids

The Coleoidea, a subclass of cephalopod molluscs who evolved internalised shell, do not have a true endoskeleton in the physiological sense; there, 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).