10Four basic tissues—epithelial tissue, connective tissue, muscular tissue, and nervous tissue—are demonstrated in fish. Epithelia are classified into two types according to the number of the layers; simple: one cell layer thick, and stratified: many layers of cells. Epithelia are classified also according to the shape of the cells into: (a) Squamous: flat cells with flat nucleus; (b) Cuboidal: square in shape with rounded nucleus; (c) Columnar: rectangular with oval basally located nucleus; and (d) Pseudostratified: two types of cells are demonstrated with nuclei arranged at different levels. Epithelia are further classified by its apical surface specializations into microvilli and cilia. Epithelia also form the essential parts of the glands. Taste buds populate in the stratified epithelium of the integument and of the buccopharyngeal to esophageal cavity. The connective tissues serve as connecting and filling tissues, lying beneath the epithelial tissues of the skin and digestive tract. Each type of connective tissue consists of cells scattered in an extracellular matrix consisting of fibers in an amorphous ground substance. Collagenous, reticular, and elastic fibers occur in connective tissue. The various cell types of connective tissue are divided into two groups: one population of fixed cells, which includes fibroblasts and adipose cells, and another population of wandering or free cells whose presence primarily depends on the functional state of the tissue. Three types of muscle cells are present in the muscular tissue of fishes; skeletal, cardiac musculature, and smooth musculatures. Skeletal muscle fibers are multinucleated syncytia that originate and insert on the bones of the skeleton. The fish contain two types of skeletal muscle types; red and white that have different degrees of vascularization and myoglobin content. These two kinds of muscles are involved in two kinds of swimming activity. Cardiac muscle fiber consists of anastomosing and branching cardiomyocytes. At sites of end-to-end contact are the intercalated discs. The specially modified conduction pathways (Purkinje fibers) seems to be lacking in fish hearts. Smooth muscle fibers are usually form the contractile portion of the walls of most viscera and the tunica media of the arteries and veins. The cells (leiomyocytes) of smooth muscle are spindle-shaped structures without striations. In fishes a clear separation between a central and a peripheral nervous system does exist. The central nervous system (CNS) consists of the brain and spinal cord and contains the neurons and host neuroglia. Nerve impulses pass to and from the CNS over long neuronal processes called axons and dendrites.