Definition of Skeleton
The skeleton is the framework that supplies structure to the remainder of the body and helps with movement.
- The skeleton is a tough and rigid structure of the body of animals that provides defense, shape, and assistance to the body organs.
- It is composed of inorganic or organic substances or both.
- In protozoa, it is secreted by a single cell, whereas in multicellular animals it is made up of specialized cells.
Types of Skeleton
There are three main types of the skeleton in animals, hydrostatic skeleton, exoskeleton, and endoskeleton.
1. Hydrostatic Skeleton
In animals that do not have a difficult skeleton, a fluid-filled gastrovascular cavity or coelom can act as a hydrostatic skeleton. A hydrostatic skeleton supplies support and resistance to the contraction of muscles so that motility results. It is found in cnidarians, annelids, and other soft-bodied invertebrates. The sea anemone has a hydrostatic skeleton. Its cavity is filled with seawater to extend its body and tentacles.
The sea polyp closes its mouth and restricts its muscle fibers that are set up in circles around its body. The contraction of these circular muscles puts pressure on the liquid in the body cavity which pressure forces the body to keep upright stature.
In earthworms, the hydrostatic skeleton includes fluid-filled compartments separated by septa. Contraction of circular muscle triggers compartments to lengthen and contraction of longitudinal muscle causes a compartment to shorten. Rotating waves of elongation and contraction move the earthworm through the soil, aided by paired setae in each segment.
2. Exoskeleton
An exoskeleton is a solidified external covering to which internal muscles are attached. The exoskeleton is inert and non-living. It is produced by the ectoderm in animal cells. It is made up of two layers.
The epicuticle is the outermost layer. Because it is comprised of waxy lipoprotein, it is impenetrable to water and serves as a barrier to microbes in insects.
The bulk of the exoskeleton is below the epicuticle and is called the procuticle. Procuticle includes an outer layer of exocuticle and an inner layer of endocuticle. The procuticle is composed of chitin, hard, leatherlike, polysaccharide, and several types of protein. It is more solidified by sclerotization and sometimes by impregnation with calcium carbonate.
The simplest example of an exoskeleton is the shell of Mollusca, which normally includes simply a couple of pieces. Some marine Bivalvia and snails have shells made up of crystals of calcium carbonate. The shell of land snail normally lacks the hard minerals and is much lighter. The molluscan shell can grow as the animal grows and growth rings appear on the shell. The pulps of the molluscan body have a hydrostatic skeleton as well.
The most complex exoskeleton is found amongst the arthropods. The arthropods have made a range of adaptations to permit them to live and grow within their exoskeleton. The invagination of exoskeleton types, firm ridges, and bars for muscle attachment. Another modification of the exoskeleton is the development of joints. The exoskeleton is thin, soft, and flexible at joints, subsequently the joint moves easily. Other modifications of the exoskeleton consist of sensory receptors called sensilla that remain in the form of bristles, and lenses and the modification of the exoskeleton that allows gaseous exchange.
The exoskeleton in Arthropoda protects the animals against their enemies and rough environment. It likewise protects them from drying. However, it has one downside and that is animals cannot grow larger. The animal, therefore, requires to shed its exoskeleton periodically and change it with among the bigger size. This procedure is referred to as “ecdysis or moulting.”
Ecdysis is divided into four stages:
- Enzymes, secreted from hypodermal glands, start absorbing the old endocuticle. This digestion separates the hypodermis and the exoskeleton.
- The old exoskeleton is split and pores are formed.
- The digestion of the endocuticle is followed by the secretion of brand-new procuticle and epicuticle.
- Finally, the new exoskeleton is solidified by the deposition of calcium carbonate. Throughout the hardening procedure, the arthropod is vulnerable to predators and stays hidden. All these changes are controlled by the nervous system and the hormone ecdysone.
3. Endoskeleton
The endoskeleton is mostly comprised of two types of tissues, bones, and cartilage. Both bones and cartilage are types of rigid connective tissue. Both consist of living cells embedded in the matrix of a protein called collagen.
Bone:
It is the most rigid type of connective tissue. The collagen fibers of bone are hardened by the deposit of calcium phosphate. Bones supporting arms and legs consist of an outer shell of compact bone, with spongy bone in the interior. Compact bone is dense and strong and supplies an accessory site for a muscle. Spongy bone is light, rich in blood vessels, and extremely porous. The cavities of spongy bone contain bone marrow where blood cells are formed.
There are 3 types of cells related to the bone: Bone-forming cells (osteoblast), mature bone cells (osteocyte), and bone-dissolving cells (osteoclast).
Early in growth, when a bone is replacing cartilage, the osteoclasts invade and dissolve the cartilage. Then osteoblasts change it with bone. As bones grow, the matrix of bone is hardened and the osteoblasts are slowly allured within it.
Cartilage:
It is much softer than bone. It is a kind of connective tissue. It covers the ends of the bone at the joint and likewise, supports the flexible part of the nose and external ears. The living cells of cartilage are called chondrocytes. These cells secrete flexible and non-living matrix collagen that surrounds the chondrocytes. No capillary permeates into this cartilage. There are 3 primary types of cartilage.
(i) Hyaline Cartilage: It is the most abundant type in the human body. It is found at the movable joints.
(ii) Flexible Cartilage: It has a matrix containing bundles of collagen fibers. It forms the external pinnae of the ears and the epiglottis.
(iii) Fibro Cartilage: It is found predominantly in the intervertebral disks and at the insertions of ligaments and tendons.
Major Functions of Skeleton
(i) Support and shape: Bones support soft tissues and act as accessory sites for a lot of muscles and provide shape to the body.
(ii) Defense: Bones protect critical internal organs, such as the brain, spinal cord, heart, and lungs.
(iii) Movement: Skeletal muscles attached to the bones assist in moving the body.
(iv) Mineral homeostasis: Bones work as a store for calcium, phosphorus, sodium, and potassium. Through negative feedback mechanisms, bones can release or take up minerals to keep homeostasis.
(v) Blood cell production: Red and white blood cells are produced in bone marrow, a connective tissue found within specific bones.
MCQs with Answers – Skeleton, Types of Skeleton, and Functions
- What is the primary function of the skeleton?
- A. Energy storage
- B. Movement
- C. Temperature regulation
- D. Digestion
Answer: B. Movement
- How is the skeleton defined?
- A. A fluid-filled cavity
- B. A rigid structure providing support
- C. A network of muscles
- D. A system of nerves
Answer: B. A rigid structure providing support
- What are the three main types of skeletons in animals?
- A. Cartilaginous, Fluid, Spongy
- B. Hydrostatic, Exoskeleton, Endoskeleton
- C. Skeletal, Muscular, Nervous
- D. Epidermal, Dermis, Subcutaneous
Answer: B. Hydrostatic, Exoskeleton, Endoskeleton
- In which animals is the hydrostatic skeleton commonly found?
- A. Birds
- B. Insects
- C. Mammals
- D. Fish
Answer: B. Insects
- What function does the hydrostatic skeleton serve in animals?
- A. Temperature regulation
- B. Support and resistance to muscle contraction
- C. Protection of internal organs
- D. Energy storage
Answer: B. Support and resistance to muscle contraction
- What is the primary component of the exoskeleton in arthropods?
- A. Calcium carbonate
- B. Chitin
- C. Collagen
- D. Lipoprotein
Answer: B. Chitin
- What process allows arthropods to grow larger despite their exoskeleton?
- A. Sclerotization
- B. Ecdysis or moulting
- C. Ossification
- D. Exuviation
Answer: B. Ecdysis or moulting
- Which type of skeleton protects animals from enemies and environmental factors but limits their growth size?
- A. Endoskeleton
- B. Hydrostatic skeleton
- C. Exoskeleton
- D. Spongy skeleton
Answer: C. Exoskeleton
- What is the process of shedding the exoskeleton and replacing it with a larger one called?
- A. Ecdysis
- B. Ossification
- C. Calcification
- D. Moulting
Answer: A. Ecdysis
- What tissues primarily make up the endoskeleton?
- A. Muscles and ligaments
- B. Bones and cartilage
- C. Chitin and lipoprotein
- D. Epidermis and dermis
Answer: B. Bones and cartilage
- Which type of cartilage forms the external pinnae of the ears and the epiglottis?
- A. Hyaline Cartilage
- B. Flexible Cartilage
- C. Fibro Cartilage
- D. Elastic Cartilage
Answer: B. Flexible Cartilage
- What is the primary function of the endoskeleton?
- A. Protecting internal organs
- B. Providing shape to the body
- C. Energy storage
- D. Assisting in digestion
Answer: A. Protecting internal organs
- Where are red and white blood cells produced in the skeletal system?
- A. Cartilage
- B. Spongy bone
- C. Compact bone
- D. Muscles
Answer: B. Spongy bone
- Which of the following is NOT a function of the skeleton?
- A. Temperature regulation
- B. Movement
- C. Defense
- D. Energy production
Answer: D. Energy production
- What type of skeleton is characterized by a fluid-filled gastrovascular cavity or coelom?
- A. Exoskeleton
- B. Endoskeleton
- C. Hydrostatic skeleton
- D. Elastic skeleton
Answer: C. Hydrostatic skeleton
- What is the primary role of the epicuticle in the exoskeleton?
- A. Muscle attachment
- B. Water repellency
- C. Joint flexibility
- D. Gaseous exchange
Answer: B. Water repellency
- Which animal group commonly has a hydrostatic skeleton?
- A. Mollusks
- B. Insects
- C. Birds
- D. Fish
Answer: A. Mollusks
- What is the function of spiracles in the respiratory system of insects like cockroaches?
- A. Blood circulation
- B. Gas exchange
- C. Water absorption
- D. Food digestion
Answer: B. Gas exchange
- Which part of the endoskeleton is rich in blood vessels and porous?
- A. Compact bone
- B. Spongy bone
- C. Cartilage
- D. Ligaments
Answer: B. Spongy bone
- What process allows arthropods to replace their old exoskeleton with a larger one?
- A. Ossification
- B. Exuviation
- C. Calcification
- D. Sclerotization
Answer: B. Exuviation
- In which type of skeleton does the movement occur through the growth and contraction of abdominal muscles?
- A. Hydrostatic skeleton
- B. Exoskeleton
- C. Endoskeleton
- D. Elastic skeleton
Answer: A. Hydrostatic skeleton
- What is the function of tracheoles in the respiratory system of insects like cockroaches?
- A. Pumping air in and out
- B. Filtering dust particles
- C. Carrying oxygen to living cells
- D. Supporting the exoskeleton
Answer: C. Carrying oxygen to living cells
FAQs – Skeleton, Types of Skeleton, and Functions
- What is the definition of a skeleton?
- The skeleton is the framework that provides structure to the body, supporting movement and offering protection to organs.
- How many types of skeletons are there, and what are they?
- There are three main types of skeletons: hydrostatic skeleton, exoskeleton, and endoskeleton.
- What is a hydrostatic skeleton, and where is it found?
- A hydrostatic skeleton is present in animals without a hard skeleton, using a fluid-filled cavity for support. It is found in cnidarians, annelids, and soft-bodied invertebrates.
- Describe the exoskeleton and its components.
- An exoskeleton is a solidified external covering produced by the ectoderm. It consists of an outer epicuticle, which repels water, and a bulk procuticle, containing chitin, proteins, and sometimes calcium carbonate.
- What is ecdysis, and which animals undergo this process?
- Ecdysis is the process of shedding an old exoskeleton and replacing it with a larger one. Arthropods, like insects and crustaceans, undergo ecdysis.
- Explain the endoskeleton and its components.
- The endoskeleton comprises bones and cartilage, both rigid connective tissues. Bones are the most rigid, hardened by calcium phosphate, while cartilage is softer and covers joints.
- What are the major functions of the skeleton?
- The skeleton provides support and shape, defense for internal organs, facilitates movement through muscle attachment, maintains mineral homeostasis, and produces blood cells in bone marrow.
- Which type of skeleton allows movement through the growth and contraction of abdominal muscles?
- The hydrostatic skeleton enables movement through the growth and contraction of abdominal muscles.
- How does the exoskeleton protect arthropods from predators and the environment?
- The exoskeleton provides protection, prevents drying, and acts as a barrier to microbes. However, arthropods need to molt periodically to accommodate growth.
- What is the primary function of cartilage in the endoskeleton?
- Cartilage supports flexible parts, covering joints, and forms the external ears and nose.
- How does the endoskeleton contribute to mineral homeostasis?
- Bones in the endoskeleton store minerals like calcium, phosphorus, sodium, and potassium, releasing or absorbing them to maintain homeostasis.
- Which cells are associated with bone and their respective functions?
- Bone-forming cells (osteoblasts), mature bone cells (osteocytes), and bone-dissolving cells (osteoclasts) are associated with bone, contributing to its structure and maintenance.
- In which animals is the hydrostatic skeleton commonly found?
- The hydrostatic skeleton is commonly found in animals like cnidarians, annelids, and other soft-bodied invertebrates.
- What are the stages involved in ecdysis (molting) in arthropods?
- Ecdysis involves the digestion of the old endocuticle, the splitting of the old exoskeleton, secretion of a new exoskeleton, and solidification of the new exoskeleton through the deposition of calcium carbonate.
- Which type of cartilage is the most abundant in the human body, and where is it found?
- Hyaline cartilage is the most abundant type, commonly found at movable joints in the human body.
Wrap up
The skeleton, a vital framework in animals, provides structural support, defense, and shape to the body. Composed of inorganic or organic substances, it is secreted by single cells in protozoa and specialized cells in multicellular animals.
There are three main types of skeletons:
- Hydrostatic Skeleton: Found in soft-bodied invertebrates, it utilizes a fluid-filled cavity for support. In creatures like the sea anemone and earthworm, circular and longitudinal muscle contractions create motility.
- Exoskeleton: Comprising an external covering, it is produced by ectoderm cells. The complex exoskeleton in arthropods, including insects, offers protection but necessitates periodic molting for growth.
- Endoskeleton: Predominantly made of bones and cartilage, it supports the body’s structure. Bones, rigid due to calcium phosphate, provide support, while cartilage, softer, covers joints and nose.
Major Functions of the Skeleton:
- Support and Shape: Bones support tissues, acting as muscle attachment sites and shaping the body.
- Defense: Bones protect vital organs such as the brain, spinal cord, heart, and lungs.
- Movement: Skeletal muscles attached to bones aid in body movement.
- Mineral Homeostasis: Bones store and release minerals like calcium, phosphorus, sodium, and potassium to maintain balance.
- Blood Cell Production: Bone marrow, a connective tissue, produces red and white blood cells.
Understanding these skeletal types and functions provides insight into the diverse adaptations that contribute to the survival and functionality of various animal species.