206 AN INTRODUCTION TO ZOOLOGY 



dorsally to the notochord. This marks them off most clearly from 

 the Invertebrates, where the nerve cord is always solid, often double, 

 as in Lumbricus, and is almost invariably situated ventrally to the 

 alimentary canal, except its most anterior pair of ganglia, which 

 may be dorsal. The lumen of the tube in the region of the spinal 

 cord, i.e. the canalis centralis, is quite small, but in the head region 

 where the nervous matter enlarges to form a brain the canal also 

 widens considerably and forms a series of hollow cavities, the 

 ventricles, as we have observed in the frog. The central nervous 

 system makes its appearance in the embryo as a thickened band of 

 ectoderm in the mid-dorsal line known as the medullary plate, and 

 this runs from the front to the hinder end. The edges of this plate 

 rise up into medullary folds, leaving a sort of gutter, the medullary 

 groove, between them. Gradually the edges come closer together 

 until they finally meet in the middle, thus giving rise to a tubular 

 structure, the anterior end of which even at this early stage already 

 shows three vesicular enlargements, the rudiments of the three main 

 divisions of the adult brain. The general ectoderm closes over the 

 top, allowing the central nervous system to sink down. Thus from 

 the very beginning the nervous system is a tubular structure com- 

 posed of ectoderm. 



The third great distinguishing point in the anatomy of the 

 Chordata is the possession in the embryo, and sometimes, as in 

 Scyllium, throughout life of a series of paired perforations in the 

 walls of the pharynx which lead directly to the outside of the animal. 

 These openings, the gill slits or pharyngeal clefts, are not more than 

 seven in number, and may be just narrow clefts all the way, or enlarge 

 to form pouches with slit-like openings to the exterior and to the 

 pharynx. The larger part of the branchial cleft is lined with 

 entoderm, but the ectoderm turns in over the outer portion. The 

 slits are supported by more or less complex arrangements of carti- 

 laginous rods termed the gill bars, the whole constituting the 

 branchial basket. The gill slits are present throughout life in the 

 fish and are functional in connection with the respiratory exchanges, 

 their walls being thrown into a series of highly vascular folds, the 

 gills. The blood, which is supplied to the gills and removed by a 

 characteristic series of vessels, is oxygenated from, and gives up its 

 carbonic acid gas to, the surrounding water as it passes through the 

 capillaries of these gills. 



In the classes above the fish the internal gills lose their respiratory 

 function, which is taken on by another set of externally developed 

 gills in some of the Amphibia, and in the rest of the Amphibia, the 

 Reptiles, Birds and Mammals, by entirely new structures, the lungs. 

 In spite of this, however, the clefts, or at any rate some of them, 



