In the vast kingdom of Animalia, organisms exhibit a remarkable diversity in form and function. While unicellular organisms perform all life processes within a single cell, multicellular animals display a more complex organization where cells with similar structures and functions are grouped together to form tissues. The study of these tissues, or histology, reveals how they are further organized to form organs and organ systems, creating a highly coordinated and efficient organism. This chapter explores the fundamental types of animal tissues and provides a detailed look at the anatomy and morphology of representative invertebrate and vertebrate animals.

Animal tissues are broadly classified into four primary types: Epithelial, Connective, Muscular, and Neural Tissue.

Epithelial Tissue forms the protective covering and lining for all body surfaces, cavities, and hollow organs. The cells are compactly packed with very little intercellular matrix. Epithelial tissues are classified into simple epithelium (composed of a single layer of cells) and compound epithelium (composed of multiple layers). Simple epithelium is further divided based on cell shape: squamous (flattened cells, involved in diffusion), cuboidal (cube-like cells, involved in secretion and absorption), and columnar (tall, pillar-like cells, also involved in secretion and absorption). Sometimes, these cells bear cilia or microvilli. Compound epithelium serves a primarily protective function against mechanical and chemical stress.

Connective Tissue is the most abundant and widely distributed tissue in the body. Its primary function is to link, support, and connect other tissues and organs. A defining feature is that its cells are embedded in a large amount of extracellular matrix. Connective tissue is classified into three types: loose connective tissue (e.g., areolar and adipose tissue, which support epithelia and store fat), dense connective tissue (e.g., tendons and ligaments, characterized by densely packed fibers for strength), and specialized connective tissue. Specialized connective tissues include cartilage, bone, and blood. Cartilage and bone form the supportive skeletal framework of the body, while blood is a fluid connective tissue that circulates throughout the body, transporting gases, nutrients, and waste products.

Muscular Tissue is responsible for movement and locomotion. It is composed of elongated cells called muscle fibers that have the ability to contract in response to stimulation. There are three types of muscular tissue: skeletal muscle, which is attached to bones and is responsible for voluntary movements; smooth muscle, found in the walls of internal organs like the intestine and blood vessels, responsible for involuntary movements; and cardiac muscle, found exclusively in the heart, which contracts rhythmically and involuntarily to pump blood.

Neural Tissue is specialized for communication and control, coordinating the body’s functions in response to internal and external stimuli. It is composed of neurons, which are the structural and functional units that transmit nerve impulses, and neuroglia, which are supporting cells that protect and nourish the neurons.

The chapter further illustrates how these tissues are organized into organs and organ systems by examining the detailed anatomy and morphology of three representative animals: the earthworm, the cockroach, and the frog.

The Earthworm (Pheretima) is a terrestrial invertebrate belonging to the phylum Annelida. It has a long, cylindrical body with metameric segmentation. The digestive system is a straight tube running from the mouth to the anus. It has a closed circulatory system, and respiration occurs through its moist body surface. The excretory organs are segmentally arranged coiled tubules called nephridia. The nervous system consists of a ventral nerve cord. Earthworms are hermaphrodites, meaning both male and female reproductive organs are present in the same individual.

The Cockroach (Periplaneta) is an invertebrate belonging to the phylum Arthropoda, the largest phylum in the animal kingdom. Its body is covered by a hard, chitinous exoskeleton and is divided into a head, thorax, and abdomen. The head bears a pair of antennae and compound eyes. The digestive system is well-developed, and the circulatory system is of the open type. Respiration is carried out by a network of air tubes called tracheae. Excretion is performed by Malpighian tubules. Cockroaches are dioecious, with separate male and female individuals, and exhibit sexual dimorphism.

The Frog (Rana) is a vertebrate belonging to the class Amphibia, representing an organism that can live both on land and in freshwater. The body is divided into a head and trunk. Frogs have a well-developed digestive system, and respiration occurs through the skin, lungs, and buccal cavity, depending on the environment. The circulatory system is closed and features a three-chambered heart. The excretory system consists of a pair of kidneys. The nervous system and endocrine system are well-developed, allowing for complex coordination and control. Frogs are dioecious, and fertilization is external, occurring in water. The life cycle includes a larval stage (tadpole) that undergoes metamorphosis to become an adult frog.