COMPARATIVE ZOOLOGY. 



of the Oyster) is covered with cilia, or minute hair-like 

 portions of protoplasm, about -5-^$ of an inch long, which 



are incessantly mov- 

 ing. Continuous 

 with thisinner lining 

 of the body (as seen 

 on the lip), and cov- 

 ering the outside, is 

 the epidermis, or cu- 

 ticle. It is the outer 

 layer of the " skin," 



FIG. 2. Various kinds of Epithelium Cells: a, colam- which W6 can re- 



nar, from small intestine; 3, a single cell, showing mftv , p u v Kliof-pv 



nucleus; &, ciliated, from one of the small air- * or J 



tubes; <?, the same, from the windpipe, with single and in Man varies in 

 cell magnified about 200 times; c, squamous, from 



eyelid of a calf, showing changes of form, from the thickness from gp-Q 

 deep to superficial cells, 1 being the scurfc 



of an inch on the 



cheek to -^ on the sole of the foot. It is constantly wear- 

 ing off at the surface, and as constantly being replenished 

 from 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 epithelial tissue, then, is 

 simply a superficial covering, bloodless and insensible, pro- 

 tecting the more delicate parts underneath. 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 taker. 



