jimtrue.com : school : BSC2011 : CH 49: Animal Systems - Skeleton

Posted by Jim True on November 2, 2004 6:24 PM. Last Updated October 22, 2006 9:23 PM

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CH 49: Animal Systems - Skeleton

Functions

• Muscle Attachment - Provides a surface or framework for muscle attachment.
• Protection - Physical protection for importan internal organs. Surrounds and encloses critical body components. Prevents H2O loss (exoskeleton).
• Support - Frames and supports body against gravity, provides general shape to the body. Gives the body rigidity and flexibility.
• Movement - Virtually all locomotion and physical movement of body includes skeletal motion.
• Only in animals with bone:
  • Mineral Reservoir - Source of Ca and P.
  • Hemopoesis - Formation of blood cells in red marrow.
  • Energy Storage - Lipids in yellow marrow.

Structure

• There are three basic types of skeletons:
1. Hydrostatic - This is a "soft" skeleton, in which muscle attaches to the body wall (integument) and fluid pressure against integument provides semi-rigid support to body and resistance to muscle contraction.
• Occurs in P.Cnidaria, Mollusca, Nematoda and Annelida
• No skeleton per se in P.Porifera, although spicules may be arranged to form support framework.
• P.Platyhelminthes - a modified hydrostatic skelton since the body is not fluid filled.
• Muscles attached to integument contract against mesoglea.
2. Exoskeleton - A "hard" or rigid outer skeleton.
• P. Mollusca - Where present, the exoskeleton (shell) is extremely hard and rigid. Main function is protection, and the skeleton does grow with the organism.
• In all mollusks with external shells, the shell is formed by secreted CaCo3.
• P.Arthropoda - A highly variable exoskeleton with different degrees of flexibility. Does not grow with the organism.
• The exoskeleton is secreted by the epidermis and consists of several layers.
• It is mainly composed of chitin which maybe impregnated with CaCo₃.
• The exoskeleton of arthropods is highly segmented and forms two to three main body regions (or tagmas):
  • Head - Including sensory appendages (eyes and antennae) and mouthparts. In subphylum Uniramia only.
  • Thorax - Chest region, includes many "walking appendages" plus wings (class Insecta only).
  • In most arthropods, the head and the thorax form a single cephalothorax
  • The cephalothorax is typically covered by a single enlarged sheet of exoskeleton called the carapace.
  • Abdomen - In most arthropods, this region is usually the most segmented, and may or may not bear appendages.
  • Articulations (joints) - regions where adjacent segments can move relative to one another.
• In arthropods, the body regions (except the abdomen) are rarely jointed, while the appendages have numerous joints (for which the phylum is named).
• Joints in arthropods are regions where the chitin is very thin (outer procuticle absent) to allow flexibility.
3. Endoskeleton - Rigid internal skeleton. Grows with organism.
• P.Echinodermata - Formed of CaCo₃ plates called ossicles. Ossicles are variable in size and degree of fusion.
• In the Class Holothuroidea, ossicles are minute and loosely arranged, while in the class Echinoidea, they form large fused plates.
• P. Chordata, S.P. Urochordata and Cephalochordata - Fibrous endoskeletal rod (notochord) only. (Considered invertebrate chordates)
• P. Chordata, S.p. Vertebrata - Except for the class Agnatha, which possesses a fibrous endoskeleton, all other classes have endoskeletons of cartilage or bone combined with some amount of cartilage.
• There are two main divisions in the vertebrate skeleton:
1. Axial Skeleton - The bones forming the main axis of the body.
• Includes the skull, vertebral column (spine), ribs and sternum
2. Appendicular Skeleton - appendages (arms, legs, forelimbs/hindlimbs, wings, fins) plus their support structures (girdles).
• Class Agnatha lack this division
• In terrestrial bony vertebrates, skeletal joints allow a wide range of appendage motions.
• Three of the most flexible include:
1. Ball-and-socket - allows movement of the appendages in three dimensions. Found where appendages join to girdles.
2. Hinge - allows movement in a single plane. Typically occur at mid-appendage (e.g. knee, elbow).
3. Pivot - allows bones to swivel around central axis. Found in forearm bones (radius).
• These joints are referred to as diarthrotic or freely moveable.
• There are also slightly moveable joints (amphiarthrotic) which allow a very limited degree of motion, e.g. intervertebral joints.
• Finally, there are immoveable (synarthrotic) joints. All are in the cranium and are called sutures.

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