4 DIVISIONS OF CHORDATA CHAP. 
growths of the pharynx, which unite with the skin of the neck 
and form a series of perforations leading to the exterior. These 
structures are the gill-slits, and in the Fishes their walls give 
rise to vascular folds or gills. With the assumption of a terres- 
trial life, the higher Vertebrates lost their gills as functional 
organs, respiration being then performed by entirely different 
organs, the lungs. But even in these cases, the gill-shts appear 
in the embryo; and remains of one pair can usually be recognised 
in the adult state of even the highest Vertebrates. Another 
fundamental characteristic of the Chordata is given by the 
central nervous system, which lies entirely above the alimentary 
canal, just dorsal to the notochord. Not only does this position 
of the nerve-centres distinguish the Chordata from Invertebrates, 
but a further point of difference is found in the development. 
While in Invertebrates the ventral nerve-cord is formed as a 
thickening of the ectoderm or outermost layer of the embryo, 
in the Chordata the nervous system is usually formed as a longi- 
tudinal groove running medianly along the back of the embryo. 
This groove closes to form a tube of nervous matter, the cavity 
of which always persists throughout life as the “central canal” 
of the spinal chord and its anterior prolongation which con- 
stitutes the “ventricles” of the brain. 
Although the animals which are considered in this chapter are 
not admitted by all zoologists to be related to the Vertebrates, 
there can be no question that their respiratory organs closely 
resemble typical gill-slits. Since, moreover, they possess struc- 
tures which can be regarded, with a fair amount of probability, 
as agreeing in essential respects with the notochord and the 
tubular dorsal nervous system of Vertebrates, it appears justi- 
fiable to include them in the Chordata, which are then sub- 
divided into (1) HrmicHorDATA, in which a “notochord” occurs 
in the anterior end of the body only; (2) UrocHorpara (Tunicata 
or Ascidians), in which the notochord is restricted to the tail; (3) 
-CEPHALOCHORDATA (Amphioxus), in which the notochord extends 
the entire length of the body and of the head ; (4) CRANIATA, in 
which a brain is developed as an enlargement of the central 
nervous system, the notochord does not extend farther forward 
than the middle of the brain, and a vertebral column is present. 
These last are thus usually known as Vertebrata, although in dis- 
tinguishing an “ Invertebrate” from a “ Vertebrate” it is more 
