INTRODUCTION. 3 



able 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 respira- 

 tory organs (gills), which are always connected with the digestive 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. 



FIG. i. Diagram of a vertebrate, a, anus; b, brain; c, coelom; da, dorsal aorta; df, 

 dorsal fin; g, gonad; gd, genital duct; h, heart; i, intestine; I, liver; m, mouth; n, notochord; 

 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; rf, ventral fin. 



The central nervous system consists of brain and spinal cord which 

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

 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. 



