MORPHOLOGY OF HIGHER TYPES 100 



united by an intercclhihir c(;uicnt to form a continuous surface. Clood 

 examples of epithelial tissues in the earthworm are the hypodermis and 

 intestinal epithelium, the peritoneum and chlorogogen cells hning the 

 coelom, and the cells of the nephridia. In vertebrate animals the outer 

 layer of skin, the inner linings of the entire digestive system including the 

 secretory portions of the digestive glands (liver, pancreas, etc.), the 

 peritoneum which lines the coelom, the lining of all blood vessels and 

 lymph channels of the bod}^, the tubules of the kidney and the lining of 

 the ureters and urinary bladder, the tubules of the gonads (reproductive 

 bodies) or so-called germinal epithelium, and the linings of the passages 

 of the reproductive system are made of epithelium. From this formida- 

 ble list of organs, which is far from complete, the importance of epithelium 

 may be judged. 



Its functions in different situations are to protect exposed surfaces 

 from abrasion and from noxious substances, and to carry on the processes 

 of secretion, excretion and absorption. Epithelial cells may also in 

 simple animals and in some parts of highly differentiated animals receive 

 stimuli and transmit impulses. 



Types of Epithelium. — Epithelial tissues are classified with reference 

 to the form and arrangement of their constituent cells, or according to 

 certain structures which the cells may possess. Thus epithelial cells are 

 called columnar if the height of the cells (thickness of the layer) is greater 

 than their width (Fig. 72, A) ; cubical if these dimensions are nearly equal 

 {B); or squamous iC and D) if the cells are very broad and flat and 

 arranged like tiles in a floor. If the columnar epithelial cells possess 

 cilia, or flagella, or protoplasmic collars and flagella they are designated, 

 respectively, ciliated columnar (£'), flagellated columnar {F) or collared 

 epithehum (G). 



The cells of the hypodermis of the earthworm and of the ectoderm of 

 Hydra may be cubical or columnar depending upon the state of contrac- 

 tion of the animal. The cells of the tubules of the kidney of vertebrates 

 are columnar or in some instances cubical. Examples of ciliated epi- 

 thelium are found in the intestinal lining of the earthworm, the surface 

 layer of the gills of mussels, the lining of the mouth and esophagus of 

 amphibians (.frogs, toads, and salamanders), the lining of the trachea 

 and bronchi of air-breathing vertebrates and the linings of certain ducts 

 and passages of the genital system. Collared epithelium is found lining 

 the radial canals of sponges (Fig. 32), while the endoderm of Hydra and 

 other coelenterates is composed of flagellated epithelium. 



When the cells of columnar epithelium divide it is usually in a plane 

 parallel to their long axis, that is, perpendiciilar to the surface which they 

 cover. In some situations, however, as in the outer skin of vertebrates, 

 the mucous linings of mouth, rectum and certain other organs, the colum- 

 nar epithelial cells usually divide in a plane perpendicular to their long 



