INTRODUCTION 3 



parts is noticeable in muscles, skeleton, nerves, blood-vessels, and, 

 to a less extent, in the excretory organs. There is no cuticular 

 skeleton, but the outer layer of the skin may be cornified or the deeper 

 layer may give rise to ossifications (scales of fishes, etc.). 



There is an internal axial skeleton, consisting of the notochord, 

 around which are developed rings of denser material, constituting 

 a backbone or vertebral column, while in front a skull encloses the 

 brain and organs of special sense, and gives support to the primitive 

 respiratory organs (gills), which are always connected with the diges- 

 tive tract. Typically there are two kinds of appendages, each with 

 an internal skeleton. These are the unpaired or median fins, dorsal 

 and ventral, which occur only in the Ichthyopsida, and the paired 

 appendages, of which there are two pairs, anterior and posterior in 

 position. 



1 iG. I. — Diagram of a vertebrate, a, anus; b, brain; c, coelom; da, dorsal aorta; df, 

 dorsal fin; g, gonad; gd, genital duct; h, heart; t, intestine; /, liver; m, mouth; n nephridia; 

 0, oviduct; p, pancreas; pc, pericardium; pf, pectoral fin; ph, pharynx, with gill clefts; s, 

 stomach; sc, spinal cord; sp, spleen; u, ureter; va, ventral aorta; vc, vertebral column; 

 »/, ventral fin. 



The central nervous system consists of brain and spinal cord which 

 lie dorsal to the notochord, and are usually protected by arches aris- 

 ing from the vertebrae and by the roof of the skull. Eyes and ears 

 are the highest of the sense organs. The alimentary canal always 

 has a liver connected with it, and a portion of the canal just behind 

 the mouth is developed into a pharynx, from which, in the young 

 of all, gill clefts extend through to or toward the exterior. In the 

 terrestrial vertebrates these gill clefts are later replaced by lungs 

 which develop from the hinder part of the pharyngeal region. 



The blood, which always contains two kinds of corpuscles, flows 

 through a closed system of vessels. A heart, ventral to the digestive 

 tract and lying in a special cavity, the pericardium, is always present. 



