BIOLOGY Volume 2 - A Guide to General Biology - 2004

18. THE ANIMAL LOCOMOTOR AND SKELETAL SYSTEMS

Movement and Locomotion

Movement in the living world can occur at 1) THE CELLULAR LEVEL, such as cytoplasmic streaming in a Cell or the swimming of Gametes; 2) the organ level, such as the contraction of The Heart or the movement of a limb; and 3) the level of the whole Organism.

The displacement of an entire organism from one place to another is called locomotion. Plants exhibit movement at the cellular and often the organ levels, but they lack locomotor activity—that is, The ability to move the whole organism in search of Water or food. For more details on this, see Chapter 16.

In the course of evolution, a vast number of animals have developed complex locomotor systems that enable them to search for and obtain food. Only a few animals have successfully adapted to a sessile lifestyle; however, even in these organisms, certain body parts are highly mobile.

For some animals, locomotor activity is not only a way to find food but also a means of escaping from predators. Furthermore, through locomotion, animals disperse, colonize new favorable habitats, and find mates.

Locomotion requires the coordinated action of the nervous, muscular, and skeletal systems. Muscles involved in locomotion are attached to the Skeleton and are therefore called skeletal muscles. They function as machines that convert chemical energy into mechanical work. Muscles are capable of contraction, thereby driving the lever systems formed by certain BONES OF THE limbs. Thanks to the coordinated action of these levers, the animal moves. The Musculoskeletal System also maintains posture and is under the overall control of the Central Nervous system.

Specialized musculature ensures the movement of substances within the body. Cardiac Muscle (Sec. 6.5) pumps Blood throughout the body, while the contraction or relaxation of smooth muscle in the walls of Blood Vessels regulates blood flow by altering their lumen. The smooth musculature of the intestine, through wave-like contractions (peristalsis), propels food along the digestive tract (Sec. 8.3.5). These are only a few of the many such processes occurring within the body.

In this chapter, we will focus primarily on locomotor activity. We will examine two systems in detail—the skeletal and muscular systems—and then explore the types of locomotion characteristic of various organisms.

Supporting Systems

As the size of plants and animals increased in the course of evolution, the need arose for specialized structures to support their body shape. These structures are especially vital for organisms that left the aquatic environment and colonized the land. In animals, support is provided by the skeleton, whereas in plants it is provided by mechanical Tissues. The latter include collenchyma, sclerenchyma, and xylem. Turgor pressure in parenchymal Cells is also important for maintaining organ shape. The Structure and Role of these tissues are discussed in Chapters 6 and 22.

18.1. Skeletal Systems

Class="center">18.1.1. Functions of the Skeleton

The Main Functions of the skeleton are as follows:

1. Support. The vast majority of animals possess some form of supporting structures. The general layout and details of their structure depend on whether the animal inhabits water or land, moves on two or four legs, travels on the ground or through the air. Any type of skeleton serves as a rigid, compression-resistant framework for the body. It helps the body maintain a specific shape. In terrestrial organisms, the skeleton provides support for the entire body mass, counteracting gravity, and in many cases elevates the body above the ground. This facilitates terrestrial locomotion. Internal Organs are anchored and suspended from the skeleton.

2. Protection. Some animals, such as Arthropods (Sec. 18.1.3), possess an exoskeleton that protects delicate internal organs. In other animals, this function is performed by PARTS OF THE endoskeleton, or internal skeleton (Sec. 18.1.4). In humans, for example, the cranium protects the Brain and Sense Organs (sight, smell, equilibrium, and Hearing), THE Vertebral Column protects the Spinal Cord, and the Ribs AND Sternum protect the heart, Lungs, and major blood vessels.

3. Locomotion. Constructed from hard material, the skeleton serves as an attachment site for muscles. When muscles contract, parts of the skeleton act as levers, enabling various movements. In soft-bodied animals, coelomic fluid serves as the support for contracting muscles during movement (Sec. 18.1.2).

4. The skeleton may also have other functions, such as hematopoiesis (blood cell production) or serving as a storage site for calcium and phosphate (Sec. 18.2.2). There are three MAIN TYPES OF skeletons: the hydrostatic skeleton, the exoskeleton, and the endoskeleton.

18.1.2. Hydrostatic Skeleton

This type of skeleton is characteristic of soft-bodied animals. Fluid is secreted into a cavity enclosed by muscular walls; by exerting pressure on the muscles, this fluid forces them to contract in order to overcome that pressure. The muscles are not attached to any rigid structures and therefore pull only against each other when they contract. The animal maintains a definite body size and shape due to the balance of pressure between the coelomic fluid on one side and the contracting muscles on the other. Typically, muscle fibers form two layers: longitudinal and circular musculature. Movement occurs because these layers function as antagonists. In segmented animals (such as the common earthworm), this effect is localized, and only specific segments move or change shape. The Role of the hydrostatic skeleton in movement will be examined in detail using the earthworm as an example in Sec. 18.5.1.

18.1.3. Exoskeleton

This type of skeleton is a hallmark of arthropods. The exoskeleton (cuticle) is secreted by the epidermis and is acellular. It consists primarily of Chitin. This is a tough external body covering made of articulated plates or tubular structures. Chitin is a strong, lightweight material, but it can be hardened by the incorporation of specialized tanning Proteins or through calcification (especially in aquatic crustaceans). In areas of the skeleton that need to remain flexible, such as the joints between plates, chitin remains unmodified. This construction of plates and tubes connected by flexible membranes provides both protection and mobility.

Arthropods are the only group of invertebrates that possess jointed appendages consisting of levers connected by hinge-like joints. These levers are moved by flexor and extensor Muscles Attached to internal ridges of the exoskeleton (Fig. 18.1). Because chitin is permeable to water, terrestrial arthropods such as insects would be at risk of desiccation. However, this is prevented by the epicuticle—a waxy layer secreted by the glandular Cells of the epidermis. Thus, the exoskeleton not only provides support and protection for internal organs but also safeguards the body against dehydration.

Fig. 18.1. Cytology/practical/54.html">Longitudinal section of an arthropod limb, showing articulations and muscles.

For small animals such as most arthropods, a hollow tubular exoskeleton serves as a highly efficient supporting and locomotor structure; a tube can withstand significantly greater loads without bending than a solid cylinder of the same mass. However, as an animal's size and mass increase (Section 2.8.6), this design becomes less practical—to maintain adequate strength, the thickness and mass of the skeleton would have to increase to the point where it ultimately became excessively heavy and cumbersome.

Growth occurs through ecdysis (molting)—in the juvenile stages of insects (larvae, nymphs), and throughout life in crustaceans. At specific intervals, the old exoskeleton is shed (molt) to reveal a new, soft, and extensible one. The animal grows while the new cuticle is still pliable, thanks to its ability to stretch and expand, a process often accompanied by A change in body shape. Eventually, the new exoskeleton hardens. Until that happens, however, the animal remains vulnerable to predators. During this period, the skeleton cannot support body weight, and almost all movement is virtually impossible. For aquatic species, this problem is less severe, as water helps support their body mass; nevertheless, both aquatic and terrestrial animals typically seek shelter during molting to minimize the risk of predation.

From an energetic standpoint, molting is a highly costly process, with energy primarily expended on synthesizing the new exoskeleton. Furthermore, this process entails a loss of material when the old exoskeleton is discarded.

18.1.4. Endoskeleton

An internal skeleton is a defining characteristic of vertebrates, and among invertebrates, it is found only in certain cephalopods. The vertebrate skeleton possesses the following key features.

1. It is composed of bone and/or Cartilage (rather than chitin).

2. It is located inside the body, with muscles attached to its exterior (unlike the exoskeleton, to which muscles attach internally).

3. It consists of living tissue and can grow within the animal's body, thus eliminating The Need for periodic molts.

4. Individual elements of the endoskeleton, much like those of the exoskeleton, are joined by articulations, albeit of a more complex nature. There are several TYPES OF JOINTS, and the bones forming them are held in position by elastic ligaments.

The overall blueprint of the Skeletal System is virtually identical in quadrupedal and bipedal vertebrates, though there are certain differences in the mobility of the hip and shoulder. We will examine these features later in connection with specific modes of locomotion.