34 



COMPARATIVE ZOOLOGY. 



of the Oyster) is covered with cilia, or minute hairs, about 

 ■^Vit of an inch long, which are incessantly moving. Con- 

 tinuous with this in- 

 ner lining of the 

 body (as seen on the 

 lip), and covering 

 the outside, is the 

 epidermis j or cuti- 

 cle. It is the outer 

 layer of the " skin," 

 which we can re- 



Fig. 2.— Various kinds of Epithelium Cells: a, colum- HlOVe by a blister, 



Tern "o^^V'ofr'^r^ and in Mar, varies in 



r* 



inch on 



tubes; d, the same, from the windpipe, with single thickness from 

 cell magnified about 200 times; c, squamous, from 



eyelid of a calf, showing changes of form, from the of an 



deep to superficial cells, 1 being the scurf. , i 



cheek to T \r on the 



"§"0 



the 



sole of the foot. It is constantly wearing off at the sur- 

 face, and as constantly growing in the deeper portion; and 

 in the process of growth and passage outward, the cells 

 change from the spherical form to dead horny scales (seen 

 in scurf and dandruff). In the lower layer of the cuticle 

 we find the pigment cells, characteristic of colored races. 

 Neither the epidermis nor the corresponding tissue within 

 (epithelium) has any blood-vessels or nerves. The epithe- 

 lial tissue, then, is simply a superficial covering, bloodless 

 and insensible, protecting the more delicate parts under- 

 neath. Hairs, horns, hoofs, nails, claws, corns, beaks, scales, 

 tortoise-shell, the wings of Insects, etc., are modifications 

 of the epidermis. 



The next three sorts of tissue are characterized by a 

 great development of the intercellular substance, while 

 the cells themselves are very slightly modified. 



(2) Connective Tissue. — This is the most extensive tissue 

 in animals, as it is the great connecting medium by which 

 the different parts are held together. Could it be taken 



