2 INTRODUCTION. 
The Chordata are bilaterally symmetrical animals with metameric 
bodies, which agree in several features not found in the other groups. 
These are (1) a central nervous system, entirely on one side of the di- 
gestive tract; (2) the presence of gill slits in the young if not in the 
adult; (3) an unsegmented axial rod, the notochord, between the 
digestive tract and the nervous system. All of these features will be 
described later. 
There are three or four divisions of Chordata, the uncertainty 
depending upon the position to be accorded the Enteropneusta. These 
are worm-like animals, occurring in the sea and represented on our 
shores by Balanoglossus. What has been described as a notochord is 
a pocket from the digestive tract, lying in a curious proboscis above 
the mouth. 
The next division, the Tunicata, includes the (marine) ‘sea-squirts.’ 
They were long regarded as molluscs, but the discovery that the young 
have true gill slits, a nervous system on one side of the alimentary 
canal, and, above all, a notochord, placed them in the present associa- 
tion. Their young (larve) are tadpole-like, the notochord is confined 
to the tail, but later the tadpole features are lost and with them the 
tail and notochord, and the adult is a sac-like animal with no re- 
semblances to its former state, or to its allies. 
The third division, the Leptocardii, embraces Amphioxus and 
a few othermarine, fish-like animals. They were long classed as fishes, 
but are far more simple than any true fish. The body is markedly 
segmented, the gill slits are very numerous and the excretory organs 
open separately to the exterior and are vermian in character. Stomach, 
vertebre and heart are lacking and the brain and sense organs are 
very rudimentary, while jaws and paired appendages are absent. 
The last class, the Vertebrata, are most nearly related to the Lepto- 
cardii, but differ in many important respects. Thus there is always 
a skull and vertebral column; the brain is larger than the spinal cord; 
there are always nose, eyes and ears; a heart is present and the excre- 
tory organs open into a common duct on either side, with an external 
opening near the anus. 
Most of the characteristics of a vertebrate may be seen from the 
accompanying diagram. The body is bilaterally symmetrical, with 
anterior and posterior ends, dorsal and ventral sides well differentiated. 
There is no external segmentation, since the muscles are not directly 
attached to the skin, but a metameric arrangement of parts is notice- 
