THE MULTICELLULAR ANIMAL 63 



branes. Specialization for secretion and excretion leads to gland 

 formation. Glands may be unicellular and scattered here and there 

 in the surface of the cellular membrane, or they may be grouped 

 at certain points of vantage. Just as often, however, many cells 

 combine to form multicellular glands, which sink, as it were, below 

 the surface as simple or complex tubes or sacs of secreting cells, and 

 thus amplify many-fold the effective surface within a given space. 

 And finally, specialized epithelial cells form important elements of 

 sense organs — the outposts of the nervous system. (Fig. 33.) 



Obviously the larger and more complex the organism, the greater 

 is the necessity for sustaining and binding material; and therefore 

 as we ascend the animal scale we find, in general, an increase in 

 supporting tissue. Whereas in epithelia the cells themselves 

 form the major part of the tissue, in supporting tissues it is direct 

 or indirect products of the cells, known as intercellular material, 



Epitheliums^ r- Duct openings 



Gland cells' 

 ABC 

 Fig. 33. — Diagram of glands. 



or matrix, that gives character to the tissue. Thus the function 

 of connective tissue is performed chiefly by intercellular bundles of 

 fibers, and the same principle is true for the cartilage and bone form- 

 ing the internal skeletons of higher animals. 



Muscular tissue is responsible for the power of motion and 

 locomotion so characteristic of most animals, and also for the 

 necessary movements performed by the internal organs in carry- 

 ing on the various life processes. Muscle cells have in a highly 

 developed and specialized form a fundamental property of all 

 protoplasm, contractility, which they exhibit by shortening and 

 broadening when stimulated by impulses coming chiefly through 

 the nervous system. A muscle is a cooperating group of muscle 

 cells, usually bound together by connective tissue and richly sup- 

 plied with blood vessels and nerves. (Figs. 7, 32, 98, 99.) 



