THE ANIMAL KINGDOM 



445 



mesenteries (Fig. 266). The pharynx is concerned with 

 respiratory function (1) by serving as a respiratory surface, (2) 

 by developing gill slits and gills, or (3) by developing lungs. 

 The presence of gill slits in the pharynx is a constant feature of 

 the vertebrate embryo regardless of the type of respiration 

 found in the adult. The central nervous system is tubular and 

 lies entirely dorsal to the 

 alimentary canal. Its anterior 

 end is enlarged to form a brain 

 composed of five regions: (1) 

 telencephalon, (2) dienceph- 

 alon, (3) mesencephalon, (4) 

 metencephalon, and (5) 

 myelencephalon. The re- 

 mainder of the neural tube is 

 the spinal cord. The special 

 senses of taste, smell, sight, 

 hearing, and equilibration are 

 highly developed, though not 

 to the same degree in all 

 forms 



system is of the closed type. 

 Hermaphroditism is rare. 



CLASS I. CYCLOSTOMATA. 

 Jawless vertebrates with a 

 cylindrical body, lacking 

 paired appendages, ribs, scales, and true teeth. The notochord 

 persists as the axial skeleton throughout life (Fig. 267). Above 

 it lies the neural tube, supported by rudimentary cartilaginous 

 neural arches. The brain is enclosed in an imperfect skull, 

 which is firmly attached to a cartilaginous basketwork of carti- 

 lage supporting the branchial region. There is a median fin 

 supported by cartilaginous rays. The mouth is suctorial and 

 is provided with sharp cuticular spines, which with similar 

 spines on the end of the tongue are used as boring or rasping 

 organs. There is no pancreas, spleen, or swim bladder. The 

 olfactory organ is unpaired and median. The heart has one 

 ventricle and one atrium. All are destructive to fishes. 



Order 1. Hyperotreta. Hag fishes. These are the most 

 primitive vertebrates and are practically parasitic in their 

 habits. There is a single cornified spine or tooth in the roof of 



Fig. 266. — Diagram of a cross sec- 

 tion of vertebrate body, a, aorta; e, 

 mucosa, the endodermal lining of the 

 The blood circulatory alim entary tract; g, gonad; k, kidney; 



me, mesentery supporting the alimen- 

 tary canal; mu, muscularis coats of 

 alimentary canal; n, notochord; n.t., 

 neural tube; p.p., parietal peritoneum; 

 r, pleural rib; s, skin; s.m., body mus- 

 culature; v, vertebra; v.p., visceral 

 peritoneum. 



