34 
COMPARATIVE ZOOLOGY. 
of the Oyster) is covered with cilia, or minute hairs, about 
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 , or cuti¬ 
cle. It is the outer 
layer of the “skin,” 
which we can re- 
Fig. 2.—Various kinds of Epithelium Cells: a, colum- U10Ve by a blister, 
nar from small intestine; 3, a single cell, showing d in M ^ in 
nucleus; &, ciliated, from one of the small air- 
tubes; d, the same, from the windpipe, with single thickness from —1— 
cell magnified about 200 times; c, squamous, from . 8 0 
eyelid of a calf, showing changes of form, from the of an inch On the 
deep to superficial cells, 1 being the scurf. , 
cheek to T V on 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 
