34 



COMPARATIVE ZOOLOGY. 



of the Oyster) is covered witli cilia, or minnte hair-like 

 portions of protoplasm, about ^Air of ^" '"ch long, which 



are incessantly iriov- 

 ^ A ing. CoTitinnous 



withthisinnerlining 

 of the body (as seen 

 on the lip), and cov- 

 ering tlie ontside, is 

 tiie epidermis, or cu- 

 ticle. It is the outer 

 layer of the "skin," 



Fig. 2.— VaiiouskindsofEpithelinm Cells: a.colnm- whlch We CaO re- 



iiar, from small intestine; 3, a single cell, showing »j,Q,^g \^y a blister 



nncleus; 6, ciliated, fmm one of the small air- .' _ _ ' 



tubes; (f, the same, from the windpipe, with single ^jjjj Jfj Man varicS in 

 cell magnifled abont 200 times ; c, squamous, from 



eyelid of a calf, showing changes of form, from the thlCKIieSS irOin ^g-g-jy 

 deep to superficial cells, 1 being the scuril 



cheek to A on the sole of the foot 



of an inch on the 

 TT5- uu nic Duic ui Kiic iwwi.. It is constantly wear- 

 ing ofE 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 inore 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 taken 



