34 
COMPARATIVE ZOOLOGY. 
of the Oyster) is covered with cilia, or minute hair-like 
portions of protoplasm, about - B - 7T 1 Tr _ 0 f an inch long, which 
are incessantly mov¬ 
ing. Continuous 
with this inner 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,” 
which we can re¬ 
move by a blister, 
and in Man varies in 
thickness from -g-J-g- 
Fig. 2.—Various kinds of Epithelium Cells: a, colum¬ 
nar, from small intestine; 3, a single cell, showing 
nucleus; b, ciliated, from one of the small air- 
tubes; d, the same, from the windpipe, with single 
cell magnified about 200 times; c , squamous, from 
eyelid of a calf, showing changes of form, from the 
deep to superficial cells, 1 beiug the scurf. 
of an inch on the 
cheek to X V on the sole of the foot. It is constantly wear¬ 
ing off at the surface, and as constantly being replenished 
from tht 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 taken 
