Chapter 2

Shape of Animals Under Influence of =Movement=

Movement greatly affects the shape of animals; during movement, an animal faces a new environment with the change of place. During the course, it faces a great amount of external resistance. Also, the animal must be kept informed about the changes taking place around him. To overcome these problems a motile animal adapts in the following way:

·        Body becomes compact to minimize surface resistance.

·        The shape is adapted to minimize further external resistance, body becomes “Bilaterally Symmetrical” and elongated in direction of movement.

·        The anterior end or fore part of body is provided with the sensory organs like eyes, ears and nose to keep the animal informed. So they are provided with head.

·        Food catching appertain or mouth is also located over head for easy capture of prey.

On the other hand “Scosile” animals face similar environment around them, therefore they are “Radially Symmetrical” and are generally devoid of head and sence organs. Examples are Sea Cucumber and Sea Anemone.

= Conditions for Locomotion =

Three conditions are necessary for the locomotion are:

1.            Propulsion

The animal must be propelled in the direction needed.

2.            Support

The body of animal must be supported during act of locomotion against the medium over which it applies force.

3.            Stability

During locomotion, the body of animal becomes temporarily unstable and looses equilibrium but stability is restored soon by some mean.

= Musculo – Skeletal System =

Skeleton

“A rigid structure by which an animal converts force into work i.e. movement is called Skeleton.”

It may be made up of bones, cartilage or hard plates. Muscles are attached with the skeleton for movement. Sometimes skeleton also serves to protect delicate internal organs.

Types of Skeleton

Skeletons are of three types:

1.   Exoskeleton

This skeleton lies outside the body of animal. This skeleton is generally made up of hard plates called Shell. The shells are made up of different inorganic salts like Calcium Carbonate, Silika and Chitin etc. The advantage of Exoskeleton is that it not only helps in locomotion but also serves as a shelter. Disadvantage is that it restricts free movement as well as animals have to shed them time to time as they increase in size. Muscles are attached with the exoskeleton which serve to move them. Arthropods bear jointed Exoskeleton.

2.   Endoskeleton

This skeleton lies inside the body surrounded by the muscles. It is generally made up of Cartilage and bones, which are derived from Mesoderm in vertebrates. Endoskeleton is composed of long shafts and plates of Cartilage and bones with which muscles are attached.

In vertebrates Endoskeleton is further classified into two portions:

(i) Axial Skeleton

Concerned with equilibrium and consist of vertebral column, ribs and skull.

(ii) Appendicular Skeleton

Concerned with locomotion and movement and consist of bones of upper or fore limbs or hind limbs, Pelvic and Pectoral girdles. Different bones of Endoskeleton are attached with each other at Joints. These joints may be fixed or moveable.

Advantage of Endoskeleton is that which provides support more effectively and also provides protection to the internal organs.

3.   Hydrostatic Skeleton

This type of skeleton is found in lower Invertebrates like Annelids and some Mollusks like Sea Anemone. In this skeleton instead of a rigid skeleton, pressurized fluid is used as skeleton. The animal being a tubular structure contains pressurized fluid surrounded by muscles. The fluid under pressure keeps the shape of the body constant. Contraction and relaxation of muscles changes the pressure and enables the animal to move.

= Joint =

Definition

“The place where two or more bones meet is called a Joint.”

Types of Joints

There are two types of joints namely:

1.   Imoveable Joints

Examples of such joints are joints of Skull bones.

2. Moveable Joints

These joints are generally related to appendicular skeleton with which muscles are attached to produce various movements.

Moveable joints are of two varieties namely:

(i) Ball and Socket Joint

In which one of the bones forms a ball like head whereas the opposite bone forms a Socket in which ball is fitted. In this joint movement is much more extensive. Examples are hip joint and shoulder joint.

(ii) Hinge Joint

In this variety of joint one of the bone is provided generally with an over grown end which fits in a cavity over the opposite bone. Thus these joints show movement in one plane only. Examples are knee joint and Elbow joint.

Structure

Moveable joint is enclosed in a whitish tough, fibrous capsule internally lined with synovial membrane which secretes a viscous fluid called Synovial Fluid. It serves as a lubricant. Each of the bone is covered with a smooth Hyaline Cartilage, which serves to decrease the friction during movement.

= Muscles =

Definition

“A muscle is a tissue capable of contraction and relaxation.”

Types of Muscles

In vertebrates, muscles are of three types:

1.        Striated or striped or Voluntary Muscles.

2.        Unstriated or Involuntary Muscles.

3.        Striated, Voluntary or Cardiac Muscles

1.   Striated or Voluntary Muscles

These muscles are also called Skeletal Muscles as these muscles are concerned with movement and locomotion. These muscles are connected with bones by means of Tendons.

Structure

The skeletal muscle is composed of a large number of Muscle fibres, each fibre measures about 1 to 40mm in length. They are held together by connective tissue. The mass of a single fibre is called sareoplasm surrounded by sheath called sarcolemma. Sareoplasm contains a number of nuclei.

Each muscle fibre contains protein filaments called Myo fibrils. They are of two types i.e. thin proteins fibrils or Actin and thick protein fibrils called Myosin. Arrangement of these Myo fibrils produces transverse light (or A) Bands and Dark (or I) Bands respectively.

In each light band in its centre, there is a dark Z line. The relatively clear region in the dark band is called H-Band. The whole length between two Z lines is called Sarcomere.

The complex arrangement of different bands thus produces three regions i.e. region of thing filaments only, region of thick filaments only and area or region of overlapping, cross bridges project outward from the thick or Myosin filaments connecting it with Actin filaments.

Mechanism of Contraction

Skeleted muscle contracts i.e. becomes short in length due to sliding of actin filaments over myosin filaments. Thus filling the gaps of H-Band. Due to the sliding of filaments the I (or light) band and H-Band become shroter and the A (or dark) band becomes longer. The distance between Z to Z lines becomes smaller. After siding of filaments, cross bridges are attached to Actin filaments and contract due to which muscle as a whole becomes shorter and contracted. Whole process of cross bridging is energy dependent, energy is supplied by the splitting of ATP into ADP. When the ATP is totally consumed, further energy is supplied by the Ceratine Phosphate formed by the breakdown of Glycogen. On further, demand of energy, Glycogen breaks anaerobically leading to the formation of Lactic Acid. This lactic acid accumulates in muscles and causes muscle fatigue.

2.   Unstriated or Smooth or Involuntary Muscle

These muscles are called Involuntary because they function under the control of Autonomic Nervous system. These muscles are Spindle Shape i.e. pointed on both ends. Each muscle fibre contains fine Myo-fibrils but the arrangement is different from the skeletal Muscles. Generally they show no dark and light bands under microscope. They are found in viscera like alimentary canal, urinary bladder and blood vessels often in two layers i.e. longitudinal and Circular muscles.

3.   Striated Involuntary or Cardiac Muscle

These are special type of muscle found only in Heart. Microscopically these muscle are striated but they are under control of Autonomic Nervous System. A special feature of these muscle is that, the muscle fibres are interconnected. Therefore on stimulation heart contracto in one piece. The stimulus for contraction is generated by Sino-Auricular Node which discharges periodically by which the heart contracts Rhythmically. This inter connected arrangement is called Syncitium.