Endoskeleton

Definition

An internal structural skeleton, typically mineralized, that lies beneath the soft tissues and provides support, muscle attachment, and protection for internal organs. In arthropods, endoskeletal elements are usually limited to internal apodemes, tentorial arms, or phragmata—rigid cuticular invaginations that serve as muscle attachment sites—while the main structural support is the external exoskeleton. True cellular endoskeletons occur in vertebrates and some invertebrate phyla such as echinoderms.

Etymology

Greek endon (within) + skeletos (dried body, skeleton)

Example

The tentorium of a grasshopper head is an endoskeletal structure formed by invaginated cuticle that braces the head capsule and provides attachment sites for mandibular and pharyngeal muscles.

Related Terms

• Exoskeleton
• apodeme
• Tentorium
• phragma
• Cuticle
• Sclerite
• arthropod anatomy

Usage Notes

In entomology, 'endoskeleton' usually refers to these internal cuticular specializations rather than a true cellular bone. Contrast sharply with the dominant exoskeletal architecture of arthropods. The term is sometimes used loosely for any internal supporting structure, but specialists reserve it for rigid, mineralized or heavily sclerotized frameworks.