Skeletal systems provide structure, protection, and surfaces for muscle attachment. There are two main types - rigid skeletons composed of bone or cartilage, and hydrostatic skeletons that use body fluids. Rigid skeletons are either exoskeletons found in invertebrates like insects, or endoskeletons in vertebrates. Endoskeletons are composed of bone, cartilage, and in early vertebrates, the notochord. Bones provide structure, protection, and storage of minerals. Cartilage is softer but still provides structure. The vertebrate skeleton consists of an axial and appendicular skeleton, with many variations between types of vertebrates.

2. Skeletal Systems
• Skeletons are supportive systems that provide:
1. Rigidity to the body
2. Surfaces for muscle attachment
3. Protection for vulnerable body organs
• Bone of the vertebrate skeleton is only one of several
kinds of supportive and connective tissues serving
various binding and weight-bearing functions
• There are two forms of skeletal systems:
1. Hydrostatic skeletons
2. Rigid Skeletons

3. Hydrostatic Skeletons
• Not all skeletons are rigid
• Many invertebrate groups use their body fluids as an
internal hydrostatic skeleton
• Muscles in the body wall of the earthworm have no
firm base for attachment but develop muscular force
by contracting against the coelomic fluids
• Earthworms and other annelids are helped by septa
that separate the body into more or less independent
compartments
• An obvious advantage is that if a worm is punctured or
even cut into pieces, each part can still develop
pressure and move
• Worms that lack internal compartments are rendered
helpless if body fluid is lost through a wound

5. • There are many examples in the animal kingdom of
muscles that not only produce movement but also
provide a unique form of skeletal support
• The elephant’s trunk is an excellent example of a
structure that lacks any obvious form of skeletal
support, yet is capable of bending, twisting, elongating,
and lifting heavy weights
• The elephant’s trunk, tongues of mammals and
reptiles, and tentacles of cephalopod molluscs are
examples of muscular hydrostats
• Like the hydrostatic skeletons of worms, muscular
hydrostats work because they are composed of
incompressible tissues that remain at constant volume
• The remarkably diverse movements of muscular
hydrostats depend on muscles arranged in complex
patterns

7. Rigid Skeletons
• Rigid skeletons consist of rigid elements, usually
jointed, to which muscles can attach
• Muscles can only contract; to be lengthened they must
be extended by the pull of an antagonistic set of
muscles
• Rigid skeletons provide the anchor points required by
opposing sets of muscles, such as flexors and extensors
• There are two principal types of rigid skeletons:
1. Exoskeleton: Molluscs, Arthropods and many other
invertebrates
2. Endoskeleton: Echinoderms and vertebrates

9. Invertebrate Exoskeleton
• May be mainly protective
• May also perform a vital role in locomotion
• An exoskeleton may take the form of a shell, a spicule,
or a calcareous, proteinaceous, or chitinous plate
• It may be:
 Rigid, as in molluscs
 Jointed and movable, as in arthropods
• Unlike an endoskeleton, which grows with the animal,
an exoskeleton is often a limiting coat of armor that
must be periodically molted to make way for an
enlarged replacement
 Some invertebrate exoskeletons, such as the shells of
snails and bivalves, grow with the animal

10. Spicules in Sponges
Chitinous Exoskeleton of Insects

11. Vertebrate Endoskeleton
• Formed inside the body
• Is composed of bone and cartilage
 Forms of dense connective tissue
• Bone not only supports and protects but is
also the major body reservoir for calcium and
phosphorus
• In amniote vertebrates red blood cells and
certain white blood cells are formed in the
bone marrow

12. Notochord
• Semirigid supportive axial rod of the
protochordates and all vertebrate larvae and
embryos
 Composed of large, vacuolated cells and is
surrounded by layers of elastic and fibrous
sheaths
 It is a stiffening device, preserving body shape
during locomotion
 Except in the jawless vertebrates (lampreys and
hagfishes), the notochord is surrounded or
replaced by the backbone during embryonic
development

14. Cartilage
• Major skeletal element of some vertebrates
 The jawless fishes and the elasmobranchs
(sharks, skates, and rays) have purely
cartilaginous skeletons
 Other vertebrates as adults have principally bony
skeletons with some cartilage interspersed
• Cartilage is a soft, pliable, characteristically deep-
lying tissue
 Cartilage is the same wherever it is found

15. Cartilaginous skeleton of shark

16. • Hyaline cartilage: The basic form, has a clear,
glassy appearance
 Composed of cartilage cells (chondrocytes)
surrounded by firm complex protein gel
interlaced with a meshwork of collagenous fibers
 Blood vessels are virtually absent (injuries
involving cartilage heal poorly)
 Makes up the articulating surfaces of many bone
joints of most adult vertebrates and the
supporting tracheal, laryngeal, and bronchial
rings

18. Cartilage similar to hyaline cartilage occurs in some
invertebrates, (A) the radula of gastropod molluscs (B)
lophophore of brachiopods
B

19. Cartilage of cephalopod molluscs is of a special type with long, branching processes
that resemble the cells of vertebrate bone
Cells of Vertebrate Bone

20. Bone
• A living tissue that has significant deposits of inorganic calcium salts laid
down in an extracellular matrix
• Its structural organization is such that bone has nearly the tensile strength
of cast iron, yet is only one-third as heavy
• Bone is never formed in vacant space but is always laid down by
replacement in areas occupied by some form of connective tissue
1. Most bone develops from cartilage and is called endochondral (“within
cartilage”) or replacement bone
 Embryonic cartilage is gradually eroded leaving it extensively
honeycombed
 Bone-forming cells then invade these areas and begin depositing calcium
salts around strand like remnants of the cartilage
2. A second type of bone is intramembranous bone
 Develops directly from sheets of embryonic cells
 Dermal bone is a type of intramembranous bone
 In tetrapod vertebrates intramembranous bone is restricted mainly to
bones of the face, cranium and clavicle; the remainder of the skeleton is
endochondral bone

21.  Whatever the embryonic origin, once fully formed,
endochondral and intramembranous bone look the
same
 Fully formed bone, however, may vary in density
• Cancellous (or spongy) bone consists of an open,
interlacing framework of bony tissue, oriented to give
maximum strength under the normal stresses and
strains that the bone receives
 All bone develops first as cancellous bone
• Some bones, through further deposition of bone salts,
become compact
 Compact bone is dense, appearing solid to the unaided
eye
• Both cancellous and compact bone are found in the
typical long bones of tetrapods

22. Microscopic Structure of Bone
• Compact bone is composed of a calcified bone matrix arranged in
concentric rings
 Rings contain cavities (lacunae) filled with bone cells (osteocytes),
which are interconnected by many minute passages (canaliculi)
 These passages serve to distribute nutrients throughout the bone
• Entire organization of lacunae and canaliculi is arranged into an
elongated cylinder called an osteon (also called haversian system)
• Bone consists of bundles of osteons cemented together and
interconnected with blood vessels and nerves
• Bone is a living tissue: Blood vessels and nerves throughout
 Nonliving “ground substance” predominates
 As a result of its living state, bone breaks can heal, and bone
diseases can be as painful as any other tissue disease

24. • Bone growth is a complex restructuring process,
involving both its destruction internally by bone
resorbing cells (osteoclasts) and its deposition
externally by bone building cells (osteoblasts)
• Both processes occur simultaneously so that the
marrow cavity inside grows larger by bone resorption
while new bone is laid down outside by bone
deposition
• Bone growth responds to several hormones:
 Parathyroid hormone from the parathyroid gland
 Stimulates bone resorption
 Calcitonin from the thyroid gland
 Inhibits bone resorption
 These two hormones, together with a derivative of
vitamin D, are responsible for maintaining a constant
level of calcium in the blood

26. Plan of the Vertebrate Skeleton
• The vertebrate skeleton is composed of two main divisions:
1. Axial skeleton: Skull, vertebral column, sternum, and ribs
2. Appendicular skeleton: Limbs (or fins or wings) and pectoral and
pelvic girdles
• Skeleton has undergone extensive remodeling in the course of
vertebrate evolution
• The move from water to land forced dramatic changes in body form
• With increased cephalization, the further concentration of brain,
sense organs, and food-gathering and respiratory apparatus in the
head, the skull became the most intricate portion of the skeleton
• Some early fishes had as many as 180 skull bones
• Skull bones became greatly reduced in number during evolution of
the tetrapods
 Amphibians and lizards have 50 to 95, and mammals, 35 or fewer,
Humans have 29

28. Vertebral Column
• Main stiffening axis of the postcranial skeleton
• In fishes it serves much the same function as the notochord
from which it is derived
 Provides points for muscle attachment and prevents
telescoping of the body during muscle contraction
• With evolution of amphibious and terrestrial tetrapods, the
vertebrate body was no longer buoyed by the aquatic
environment
• Vertebral column became structurally adapted to withstand
new regional stresses transmitted to the column by the two
pairs of appendages
• In amniote tetrapods (reptiles, birds, and mammals), the
vertebrae are differentiated into cervical (neck), thoracic
(chest), lumbar (back), sacral (pelvic), and caudal (tail)
vertebrae

30. • In birds and humans the caudal vertebrae are reduced
in number and size, and the sacral vertebrae are fused
• The number of vertebrae varies among the different
vertebrates:
 Pythons seems to lead the list with more than 400
 In humans there are 33 in a young child, but in adults 5
are fused to form the sacrum and 4 to form the coccyx
 Besides the sacrum and coccyx, humans have 7
cervical, 12 thoracic, and 5 lumbar vertebrae
• Number of cervical vertebrae (7) is constant in nearly
all mammals
 The first two cervical vertebrae, atlas and axis, are
modified to support the skull and permit pivotal
movements
 The atlas bears the globe of the head
 The axis permits the head to turn from side to side

32. Ribs
• Long or short skeletal structures that articulate medially with
vertebrae and extend into the body wall
• Fishes have a pair of ribs for every vertebra
 They serve as stiffening elements in the connective tissue septa
that separate the muscle segments and thus improve the
effectiveness of muscle contractions
 Many fishes have both dorsal and ventral ribs, and some have
numerous rib-like intermuscular bones as well—all of which
increase the difficulty and reduce the pleasure of eating certain
kinds of fish
• Other vertebrates have a reduced number of ribs, and some, such
as the familiar leopard frog, have no ribs at all
• In mammals the ribs together form the thoracic basket, which
supports the chest wall and prevents collapse of the lungs
• Mammals such as sloths have 24 pairs of ribs, horses posses 18
pairs, primates other than humans have 13 pairs of ribs; humans
have 12 pairs

34. Intermuscular bones are small, hard spicules of bone

35. Frog: No ribs at all Humans: 12 pairs of ribs

36. Appendages
• Most vertebrates, fishes included, have paired appendages
• All fishes except agnathans have thin pectoral and pelvic fins that are
supported by the pectoral and pelvic girdles, respectively
• Tetrapods (except caecilians, snakes, and limbless lizards) have two
pairs of pentadactyl (five toed) limbs, also supported by girdles
Caecilian Legless lizards

37. • The pentadactyl limb is similar in all tetrapods, alive
and extinct
 Even when highly modified for various modes of life,
the elements are rather easily homologized
• Modifications of the basic pentadactyl limb for life in
different environments involve distal elements much
more frequently than proximal
 It is far more common for bones to be lost or fused
than for new ones to be added
 Horses and their relatives evolved a foot structure for
fleetness by elongation of the third toe
 In effect, a horse stands on its third fingernail (hoof),
much like a ballet dancer standing on the tips of the
toes

39. • The bird wing is a good example of distal modification
• Bird embryo bears 13 distinct wrist and hand bones
(carpals and metacarpals)
 These are reduced to three digits in the adult
 Most finger bones (phalanges) are lost, leaving four bones
in three digits
 The proximal bones (humerus, radius, and ulna), however,
are only slightly modified in the bird wing
• In nearly all tetrapods the pelvic girdle is firmly attached to
the axial skeleton
• The greatest locomotory forces transmitted to the body
come from the hindlimbs
• The pectoral girdle is much more loosely attached to the
axial skeleton, providing the forelimbs with greater
freedom for manipulative movement