Bio Review Notes #44
SKELETON IN GENERAL
Performance Objectives:
Various types of skeletal system can help support the body's shape. Fluid-filled cavities can form a hydrostatic skeleton. Skeletons can also be built of rigid structures on the outside (exoskeletons) or on the inside (endoskeletons). Skeletal proportions (and therefore body proportions) are often dictated by area/volume relationships.

Hydrostatic skeleton (as in many worms): a series of fluid-filled body cavities (coelomic cavities) can form a skeleton. Such a skeleton can be very useful in burrowing through loose sand of soil: those segments contricted by circular fibers protrude forward and lengthen the body, while segments compressed along the body axis by longitudinal fibers bulge sideways and anchor those parts of the body against slipping backward.

Exoskeleton (as in insects and other arthropods): body parts are hollow, with hardened tissue on the outside. In small organisms (most insects, spiders), exoskeletons can be made of chitin, but in larger organisms (lobsters), chitin layer is usually strengthened by calcium salts.
  • Muscles are arranged on the inside, spanning joints. Most muscles span one joint at a time.
  • The weight of the exoskeleton limits its size, especially on land.
  • Growth is a problem once the skeleton hardens, so animals with exoskeletons molt at certain intervals: the animal cracks through and sheds its skeleton, exposes a new skeletal layer underneath, inflates its body, and allows its new, larger skeleton to harden.

Endoskeleton: a skeleton on the inside, as in vertebrates. Muscles are arranged surrounding the bones of the skeleton. Animals with endoskeletons are generally larger than those with exoskeletons. Growth is less of a problem because skin can remain flexible.

Area-volume relationships: Skeletal proportions are often dictated by area-volume relationships, which limit body shapes and sizes.
  • Mass and muscle strength are both related to volume.
  • Efficiency of digestion, respiration, or heat exchange are related to surface area, so large animals need more heavily folded surfaces.
  • Strength of supporting legs is related to cross-sectional area, while the weight needing support is related to volume. Thus, large animals (elephants, brontosaurs) need disproportionately thicker, cylindrical, and more vertical legs.

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