2 INTRODUCTION 



The Chordata are bilaterally symmetrical animals with meta- 

 meric bodies, which agree in several features not found in the other 

 groups. These are (i) a central nervous system, entirely on one 

 side of the digestive 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 association. Their young (larvae) 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 resemblances to its former state, or to its allies. 



The third division, the Leptocardii, embraces Amphioxus and 

 a few other marine, fish-like animals. They were long classed as 

 fishes, but they are far more simple than any true fish. The body is 

 markedly segmented, the gill slits are very numerous and the ex- 

 cretory organs open separately to the exterior and are vermian in 

 character. Stomach, vertebrae 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 

 Leptocardii, 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 excretory 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 difieren- 

 tiated. There is no external segmentation, since the muscles are 

 not directly attached to the skin, but a metameric arrangement of 



