39.1 Diversity of Skeletons
1. Three types of skeletons occur in the animal kingdom.
2. A hydrostatic skeleton occurs in cnidarians, flatworms, roundworms, and annelids.
3. An exoskeleton is found in molluscs and arthropods.
4. An endoskeleton is found in sponges, echinoderms, and vertebrates.
A. Hydrostatic Skeleton
1. A fluid-filled gastrovascular cavity or coelom can act as a hydrostatic skeleton.
2. It offers support and resistance to the contraction of muscles for motility.
3. Many animals have hydroskeletons.
a. Hydras use a fluid-filled gastrovascular cavity to support tentacles that rapidly contract.
b. Planarians easily glide over substrate with muscular contractions of body walls and cilia.
c. Roundworms have a fluid-filled pseudocoelom and move when their longitudinal muscles contracct against it.
d. Earthworms are segmented with septa dividing the coelom into compartments; circular and longitudinal muscles contract in each segment to coordinate elongation and contraction.
4. Animals with exoskeletons or endoskeletons move selected body parts by means of muscular hydrostats, i.e., fluid contained within certain muscle fibers assists movement of that part.
B. Exoskeletons and Endoskeletons
1. An exoskeleton is an external covering composed of stiff material.
a. Molluscs have exoskeletons that are predominantly calcium carbonate.
b. Insects and crustacea have jointed exoskeletons composed of chitin, a strong, flexible, nitrogenous polysaccharide.
c. The exoskeleton provides protection against damage from enemies and also keeps tissues from drying out.
d. Although stiffness provides support for muscles, the exoskeleton is not as strong as an endoskeleton.
e. The clam and snail exoskeletons grow with the animals; their thick nonmobile calcium carbonate shell is for protection.
f. The chitinous exoskeleton of arthropods is jointed and moveable.
g. Arthropods must molt when their exoskeleton becomes too small; a molting animal is vulnerable to predators.
2. Vertebrates have an endoskeleton composed of internal bone and cartilage that grows with the animal.
a. The endoskeleton does not limit the space available for internal organs and it can support greater weight.
b. Soft tissues surround the endoskeleton to protect it; injuries to soft tissue are easier to repair.
c. Usually an endoskeleton has elements that protect vital internal organs.
d. The jointed exoskeleton of arthropods and endoskeletons of vertebrates allow flexibility and helped arthropods and vertebrates colonize land.
39.2 The Human Skeletal System
1. The skeletal system has the following functions that contribute to homeostasis:
a. Support of the body.
b. Protection of vital internal organs.
c. Site for muscle attachment.
d. Storage reservoir for ions.
e. Production of blood cells.
A. Bone Growth and Renewal
1. The prenatal human skeleton is cartilaginous; cartilage structures serve as “models” for bone construction.
a. The cartilaginous models are converted to bones when calcium salts are deposited in the matrix, first by cartilaginous cells and later by bone-forming cells called osteoblasts.
b. Conversion of cartilaginous models to bones is called endochondral ossification.
c. Some bones (e.g., facial bones) are formed without a cartilaginous model.
2. During endochondral ossification, there is a primary ossification center at the middle of a long bone; latter secondary centers form at the ends.
3. A cartilaginous growth plate occurs between primary and secondary ossification centers.
4. As long as the growth plate remains between the two centers, bone growth occurs.
5. The rate of growth is controlled by hormones, including growth hormone (GH) and sex hormones.
6. Eventually plates become ossified and bone stops growing; this determines adult height.
7. In adults, bone is continually being broken down and built up again.
a. Bone-absorbing cells (osteoclasts) break down bone, remove worn cells, and deposit calcium in the blood.