70 



THE ORGANIZED ANIMAL 



Fig. 4-3. Various kinds of glands that arise from epithelial tissue. A. A simple flask-shaped gland (alveolar) such 

 OS that found in the skin of the frog used to secrete mucus. B. A compound alveolar gland such as that found 

 in the salivary glands. C. A simple tubular gland such as that in the lining of the intestine. D. A compound 

 tubular gland such as that found in the stomach lining. 



Once cells have established themselves in 

 such intricate relationship as in Hydra, fur- 

 ther differentiation is possible. That pos- 

 sibility becomes a reality as we go higher 

 in the animal series. The result is a number 

 of different kinds of cells, each carrying on 

 its own metabolism, but specialized for 

 some particular function. The major types 

 of such specialization are not great in num- 

 ber but each type has many varieties. Let 

 us examine these major forms of specializa- 

 tion. 



ORGANIZATION OF CELLS 

 INTO TISSUES 



Division of labor among the soma cells 

 spread throughout the Metazoa until a wide 

 variety of cells was produced, each doing 

 a specific job. Cells of the same kind grouped 

 together in a continuous mass form a tissue. 

 A particular kind of tissue is not necessarily 

 limited to one region of an animal body, 

 but usually is found in several different 

 places, where it may or may not perform 

 the same function. There are four major 

 kinds of tissues, epithelial, sustentative, 

 nervous, and contractile. Although tissues 

 occur in all groups of animals, for simplicity 

 let us consider only those found in a mam- 

 mal such as man. 



Epithelial tissues. These are the surface 

 tissues which cover and line not only the 



outside of the body but the cavities within 

 as well. They are composed of closely 

 fitting cells forming continuous membranes 

 and with very little intercellular material 

 binding them together. Since the jobs per- 

 formed by epithelial tissues vary greatly, 

 these tissues exhibit a wide variety of form 

 (Fig. 4-2). The tissue is usually named ac- 

 cording to the shape of its constituent cells, 

 for example, squamous (flat), cuboidal 

 (cubes), and columnar (columns or pil- 

 lars ) . They may also be described in terms 

 of accessory structures such as flagella, 

 collars, or cilia. Finally, the tissue may be 

 referred to as stratified if the cells have dif- 

 ferent forms and lie several cells in thick- 



ness. 



In addition to protection, epithelial cells 

 that line cavities usually have the function 

 of secretion, which is the production of spe- 

 cial substances used by the organism in 

 various ways. For example, the cells lining 

 the digestive tract are mostly secretory in 

 function. These cells form the secreting por- 

 tion of glands whether the gland is single 

 or many celled. In multicellular glands the 

 secreting cells may lie beneath the sur- 

 rounding surface forming simple tubes 

 (tubular glands) or flask-shaped pockets 

 (alveolar glands). Such tubes and pockets 

 may be single structures (simple glands) 

 or they may be grouped into aggregates 

 (compound glands) (Fig. 4-3). 



