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Types of skeleton. Prepared By : Sadia khurshid Lecturer , Zoology Department DA Degree College for Women, Phase VIII . Content. INTRODUCTION TYPES OF SKELETON HYDROSTATIC SKELETON EXOSKELETON ENDOSKELETON FACTS REFERENCES. The Skeletal System.

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types of skeleton

Types of skeleton

Prepared By :

Sadia khurshid

Lecturer , Zoology Department

DA Degree College for Women, Phase VIII

the skeletal system
The Skeletal System

The system which is consist of supporting tissues. This is to protects the body or a part of body and play an important role in animal’s physiology.

Fig.1. human skeleton

functions of skeletal system
Functions of skeletal system
  • Support and shape
  • Protection
  • Movement
  • Mineral homeostasis (Ca, P, Na, & K)
  • Blood cell production
  • The skeleton is a tough & rigid framework of body of animals which provides protection , shape, and support to the body organs.
  • It is composed of organic & inorganic material or both. There are three types of skeleton:
  • Hydrostatic skeleton
  • Exoskeleton
  • Endoskeletons.


1 hydrostatic skeleton
1. Hydrostatic Skeleton
  • It consist of fluid-filled closed chambers. Internal pressures generated by muscle contractions cause movement as well as maintain the shape of the animals, such as the sea anemone and worms.

Fig. 3. jelly fish

hydrostatic skeleton
Hydrostatic skeleton
  • In animal that lack the hard skeleton, a fluid filled cavity act as a hydrostatic skeleton.
  • It provide support & resistance to the contraction of muscles so that mobility results.
  • It is found in:
  • Cnidarians: jelly fish, sea anemone,
  • Annelides: earthworms
  • Molluscs: octopus

Fig. 4

Octopus octopus marginatus

Fig. 5

A Neries

hydrostatic skeleton of sea anemone
Hydrostatic skeleton of sea anemone
  • Skeleton is used to extend its body & tentacles.
  • Its body cavity is filled with sea water.
  • The sea anemone has one set of longitudinal muscles in the outer layer of the body, and a layer of circular muscles in the inner layer of the body.

The anemone can elongate or contract its body by contracting one or the other set of muscles.

FIG. 6 sea anemone

hydrostatic skeleton of earthworm
Hydrostatic skeleton of earthworm
  • The hydrostatic skeleton consist of fluid filled compartments separated by septa.
  • Contraction of circular muscles cause compartments to elongate. and contraction of longitudinal muscles causes a compartment to shorten.
  • Setae helps in locomotion.
  • When the pressure of coelomic fluid is decreased its result in a relaxation of muscles.

Fig. 7 Earth worm


Successive peristaltic waves begin at the front and move towards the end of the body

  • Earthworm has bristle (chaetae)anchors part of the body to the ground so that other parts can be pulled towards the static parts.

Fig. 8 Movement of earth worm

hydrostatic skeleton of octopus
Hydrostatic skeleton of Octopus
  • The mollusc possess a coelom, a fluid-filled cavity that develops within the mesoderm. 
  • The coelom not only functions as a hydrostatic skeleton but also provides space within which the internal organs can be suspended by the mesenteries.

Fig. 9 Octopus

2 exoskeleton
  • Exoskeletons is hardened outer surface to which internal muscles can be attached.
  • It is inert & non living.
  • It is secreted by ectoderm in animal cells.
  • It consist of epicuticle, procuticle & endocuticle.

Fig. 10 An insect

shelled protozoans
Shelled Protozoans
  • The smallest exoskeletons are found on microscopic animals such as diatoms and certain protozoan's.
  • Can push a pseudopods through a hole in the shell.
  • This one is called difflugia.

Fig. 11 Difflugia


Arcella is a genus of

 testate amoebae or Arcellinida, usually found in freshwaters

A key characteristic of Arcella is the circular test with a hole on its center from where finger-like pseudopods emerge. It is one of the largest testacean genera.

FIG. 12 Arcella


Foraminifera (forams for short) are single-celled protists with shells. Their shells are also referred to as tests because in some forms the protoplasm covers the exterior of the shell.

  • Coral reefs are made up of the accumulated exoskeletons of the coral polyp.
  • Some cnidarians produce skeletons of limestone known as corals.
  • These animals secrete their skeletons from their base.

Fig. 13 A typical foram : In the inside which the foram lives. Radiating from the picture about, the dark brown structure is the test, or shell, opening are fine hair like reticulopodia, which the foram uses to find and capture food.

Fig. 14 Cnidarians

shelled molluscs
Shelled Molluscs
  • It provides formidable protection.
  • It is bulky and severely restrictive of movement.
  • May be univalve or bivalve.
  • The mollusc shell is typically a calcareous exoskeleton which encloses, supports and protects the soft parts of an animal like snails, clams, tusk shells, and several other classes.
  • Having lines of growth.

Fig. 15 Shell of Mollusc

exoskeleton of insects arthropods
Exoskeleton of Insects & Arthropods
  • In insects and other arthropods, support is provided by the exoskeleton (external skeleton).
  • Exoskeletons can play a double role by helping animals to conserve water, but they have one important disadvantage: unlike an internal bony skeleton, their weight increases very rapidly as they get bigger, eventually making them too heavy to move.
  • This explains why insects have all remained relatively small, while some vertebrates have reached very large sizes.
advantage of exoskeleton in arthropods
Advantage of exoskeleton in Arthropods
  • It forms ridges & bars for the muscle attachment.
  • Exoskeletons protects the animal from hard environment, and dehydration.
  • Exoskeleton shows the formation of joints. Exoskeleton possess sensory structure called sensilla and lenses.
  • Permits exchange of gases .
  • Spiders use a combination of an exoskeleton for protection and fluid pressure for movement.
  • Animal can not grow larger.
  • To grow they shed their exoskeleton , called moulting. It consist of 4 stages.

3. Splitting of old exoskeleton

1. Secretion of enzyme, removal of endocuticle & separation of skin from the exoskeleton

4.Hardening of new exoskeleton by deposition of calcium carbonate

2. Formation of new epicuticle & procuticle.


Step 1 (cicad)

Step 2

Fig. 16

Step 3

Step 4

  • Endoskeletons is the internal support structure of an animal, composed of mineralized tissue and are typical of many vertebrates.

Fig. 17 Endoskeleton

endoskeleton of sponges
Endoskeleton of sponges
  • The skeleton of sponges consists of microscopic calcareous or silicious spicules.
  • The demosponges include 90% of all species of sponges. Their "skeletons" are made of Spicules consisting of fibers of the protein spongin, the mineral silica, or both.
  • Where Spicules of silica are present, they have a different shape from those in the otherwise similar glass sponges.

Fig. 18 Spicules

endoskeleton of echinoderm
Endoskeleton of Echinoderm
  • The skeleton of the echinoderms, which include, the starfish, is composed of calcite and a small amount of magnesium oxide.
  • It lies below the epidermis in the mesoderm and is within cell clusters of frame-forming cells. This structure formed is porous and therefore firm and at the same time light.
  • It coalesces into small calcareous ossicles (bony plates), which can grow in all directions and thus can replace the loss of a body part. Connected by joints, the individual skeletal parts can be moved by the muscles.

Fig. 19 Ossicle of star fish

endoskeleton of vertebrates
Endoskeleton of Vertebrates
  • Vertebrates have developed an internal mineralized endoskeleton composed of bone and/or cartilage.
  • Living material of endoskeleton grows in correlation with the growth of the animal.
  • They articulate with each other at a variety of joints that allow a wide range of movements brought about by the contraction of muscles attached to them.
  • Muscles are on the outside of the endoskeleton.
living tissues of endoskeleton
Living tissues of endoskeleton
  • Cartilage and bone are types of connective tissue.
  • Bones: living cells are osteocyte ,collagen fibre are hardened by calcium phosphate.
  • Cartilage: living cells are chondrocytes.
  • Sharks, and rays have skeletons composed entirely of cartilage; other vertebrates have an embryonic cartilage skeleton progressively replaced by bone as they mature and develop.
human skeleton
Human skeleton
  • An adult human endoskeleton consists of 206 bones.
  • An human endoskeleton is about 18% of the total body weight.
  • The vertebrate skeleton consists of
  • The axial skeleton :skull, vertebral column, and rib cage
  • The appendicular skeleton : limbs ,pectoral girdle and pelvic girdle.
  • The basic plan for vertebrates is similar, although large variations occur in relation to functional demands placed on the skeleton.

Axial skeleton

  • The axial skeleton supports and protects the organs of the head, neck, and torso, and in humans it comprises the skull, ear ossicles, hyoid bone, vertebral column, and rib cage.


  • The adult human skull consists of eight bones which form the cranium, or braincase, and 14 facial bones that support the eyes, nose, and jaws.
  • Lower jaw is supported by a single bone called Dentary.
vertebral column
Vertebral column
  • The vertebral column has 33(after fusion 26) individual vertebrae separated from each other by a cartilage disk. These disks allow a certain flexibility to the spinal column, although the disks deteriorate with age, producing back pain. These discs prevents the grinding of these bones.
  • The sternum is connected to all the ribs except the lower pair. Cartilage allows for the flexibility of the rib cage during breathing.
appendicular skeleton
Appendicular skeleton
  • The appendicular skeleton is composed of 126 bones in the human body. The word appendicular is the adjective of the noun appendage, which itself means a part that is joined to something larger.
  • The appendicular skeleton is divided into six major regions:
  • 1) Pectoral Girdles (4 bones) - Left and right Clavicle (2) and Scapula (2).
  • 2) Arm and Forearm (6 bones) - Left and right Humerus (2) (Arm), Ulna (2) and Radius (2) (Fore Arm).
  • 3) Hands (58 bones) - Left and right Carpal (16) (wrist), Metacarpal (10), Proximal phalanges (10), Middle phalanges (8), distal phalanges (10), and sesamoid (4).
hind limb girdle
  • 4) Pelvis (2 bones) - Left and right os coxae (2) (ilium).
  • 5) Thigh and leg (8 bones) - Femur (2) (thigh), Tibia (2), Patella (2) (knee), and Fibula (2) (leg).
  • 6) Feet (56 bones) - Tarsals (14) (ankle), Metatarsals (10), Proximal phalanges (10), middle phalanges (8), distal phalanges (10), and sesamoid (4).
  • It is important to realize that through anatomical variation it is common for the skeleton to have many extra bones (sutural bones in the skull, cervical ribs, lumbar ribs and even extra lumbar vertebrae)
a fact
  • The appendicular skeleton of 126 bones and the axial skeleton of 80 bones together form the complete skeleton of 206 bones in the human body. Unlike the axial skeleton, the appendicular skeleton is unfused. This allows for a much greater range of motion.
some interesting facts
Some interesting facts
  • The human skeleton consists of 206 bones. We are actually born with more bones (about 300), but many fuse together as a child grows up. These bones support your body and allow you to move. Bones contain a lot of calcium (an element found in milk, broccoli, and other foods). Bones manufacture blood cells and store important minerals.
  • The human skull, or cranium, has a number of individual bones tightly fitted together at immovable joints. At birth many of these joints are not completely sutured together as bone, leading to a number of "soft spots" or fontanels, which do not completely join until the age of 14-18 months.
  • The longest bone in our bodies is the femur (thigh bone). The smallest bone is the stirrup bone inside the ear. Each hand has 26 bones in it. Your nose and ears are not made of bone; they are made of cartilage, a flexible substance that is not as hard as bone.Differences in males and females: Males and females have slightly different skeletons, including a different elbow angle. Males have slightly thicker and longer legs and arms; females have a wider pelvis and a larger space within the pelvis, through which babies travel when they are born.
functions of muscles and bones
Functions of Muscles and Bones
  • The skeleton and muscles function together as the musculoskeletal system. This system (often treated as two separate systems, the muscular, and skeletal) plays an important homeostatic role: allowing the animal to move to more favorable external conditions. Certain cells in the bones produce immune cells as well as important cellular components of the blood. Bone also helps regulate blood calcium levels, serving as a calcium sink. rapid muscular contraction is important in generating internal heat, another homeostatic function.