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 Atriozoa 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 ingestive 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 skin 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 layer, 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 exoskeleton consisting of 

 local productions of horny material, such as scales, claws, 

 horns, feathers, or hairs. 



