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-slits 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) HEMICHORDATA, in which a "notochord" occurs 

 in the anterior end of the body only; (2) UKOCHORDATA (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 



