VERTEBRATA. 40 
SUB-PHYLUM II.—VERTEBRATA. 
The Vertebrata have been illustrated by no less than seven 
types taken from the six classes. They show a remarkable 
gradation in structure, which has only one break involved 
in passing from aquatic to terrestrial habitat. 
The general: characters of the Vertebrata separating them 
from the Azriogoa are as follows :— 
1. A complex skin or external covering to the body. 
2. A brain with three primary vesicles. 
3. Three pairs of cephalic sense-organs. 
4. The notochord surrounded, and in most cases 
replaced, by a mesoblastic skeleton of cartilage, and in 
higher types, of bone. 
5. The presence of ingestivé organs, in the form of jaws 
or teeth, in correlation with which the pharyngeal clefts are 
purely respiratory (gill-slits) and the endostylar apparatus 
becomes vestigial. 
6. In all but the lowest class there are two pairs of paired 
limbs and a series of cartilaginous visceral arches. 
ORGANS OF VERTEBRATA. 
We may now briefly review the chief organs of Vertebrata. 
Skin.—The shiz is formed of two distinct parts termed 
the epidermis and dermis. The epidermis is formed of a 
basal epithelium resting upon the dermis, which represents 
the primary epiblastic layer of the embryo and of a mass 
of cells above it which have been produced by prolifera- 
tion. This mass can be defined as consisting of a lower 
portion of growing cells, called the mucous Jayer, and an 
upper superficial layer of compressed horny cells, called the 
corneous layer. 
The dermis is derived from the mesoblast and is formed 
of connective tissue and muscle intersected by nerves and 
blood-vessels. 
There are usually skin-glands formed from the mucous 
layer, and there is commonly an exoskelefon consisting of 
local productions of horny material, such as scales, claws, 
horns, feathers, or hairs. 
