668 - Heredity and Evolution 



blocks, or segments, the vertebrae. Each 

 vertebra, typically, displays an archlike sec- 

 tion, the neural arch, which grows dorsally 

 around the nerve cord. Thus all the verte- 

 brae together form a tunnellike protection 

 for the spinal cord. Moreover, a protective 

 encasement, the cranium, composed of either 

 cartilage or bone, always develops around 

 the anterior larger part of the nerve cord, 

 which later becomes the brain. In fact, some 

 zoologists prefer to designate vertebrate ani- 

 mals as Craniata. 



All organs and organ systems throughout 

 the vertebrate group display a progressive 

 series of homologies. (1) The jaws, which are 

 present in all except the lowest group (p. 

 668), are always derived from skeletal pieces 

 present in the gill arches (between the gill 

 slits). (2) The digestive tract has two major 

 glands, the liver and pancreas, which arise as 

 outgrowths from the intestinal part of the 

 enteron. (3) The skin, or integument, always 

 has a stratified epithelium, and always con- 

 sists of both ectoderm (the epidermis) and 

 mesoderm (dermis). (1) F.xcept for the lower 

 forms, which have ten, Vertebrata have twelve 

 pairs of cranial nerves, each with a charac- 

 teristic pattern of connections with the vari- 

 ous receptors and effectors in the head region, 

 and there is one pair of double-rooted spinal 

 nerves for each segment of the body. (5) The 

 heart is always derived from a major blood 

 vessel in the anterior ventral part of the body 

 and shows a progressive development from 

 the two-chambered structure of the fish to 

 the three- and four-chambered form of the 

 higher groups. (6) The arterial system dis- 

 plays a number of aortic arches (Fig. 17-14), 

 leading from (he ventral to the dorsal aorta. 

 (7) The aortic arches vary from five to six 

 pairs in fish down to one single arch in birds 

 and mammals. (8) Erythrocytes, bearing hemo- 

 globin, are always present in the blood. (9) 

 The lungs, when present, originate as out- 

 growths from the pharynx. (10) Two pairs of 

 jointed appendages, whether fins, wings, or 

 legs, are highly characteristic, and the ap- 

 pendages, which are all homologous as to 



their skeletal structure (Fig. 29-1), are sus- 

 pended to the axial skeleton by girdles, the 

 pectoral girdle anteriorly, and the pelvic 

 girdle posteriorly. (11) The excretory organs 

 and gonads arise in the dorsal region of the 

 coelomic cavity and show a progressive series 

 of modifications from lower to higher groups. 

 (12) The endocrine glands and their manner 

 of origin are similar throughout. In fact, 

 homologies among Vertebrata are almost 

 limitless, if all phases of the comparative 

 anatomy and embryology are explored com- 

 pletely. 



Classes of Vertebrata. (Fig. 32-39). Living 

 vertebrates usually are subdivided into seven 

 groups: Class 1: The Cyclostomata (literally, 

 "round-mouthed"); Class 2: The Chondrich- 

 thyes (literally, "cartilaginous fish"); Class 3: 

 The Osteichthyes (literally, "bony fish"); Class 

 4: The Amphibia (literally, "double-lived"); 

 Class 5: The Reptilia (reptiles); Class 6: The 

 Avcs (birds); and Class 7: The Mammalia 

 (mammals). Also there are two well-estab- 

 lished extinct classes: (1) the ancient jawless 

 fishes (Ostracadermi), which did not have 

 any jaws and usually possessed a heavy armor 

 of large fused scales, covering the head and 

 front part of the body; and (2) the ancient 

 jawed fish (Placodermi), which had very 

 primitive, incomplete jaws and (usually) a 

 lighter armor composed of smaller scales. 



The Cyclostomata (lampreys, hagfishes, and 

 slime eels). These are the nrost primitive of 

 living Vertebrata. All are small (under three 

 feet long), eellike, aquatic animals (Fig. 32- 

 39). Cyclostomates are unique among living 

 vertebrates because they do not possess any 

 jaws. Moreover, the notochord persists in 

 the adult as a functional skeletal structure; 

 that is, the segmented vertebral column is 

 represented only by a series of cartilaginous 

 neural arches protecting the spinal cord. 

 There are no scales, paired fins, or skeletal 

 girdles. Frequently the Cyclostomata and 

 Ostracodermi are grouped together in a sepa- 

 rate category, the Agnatha (literally, "with- 

 out jaws"). 



Probably the most familiar of the cyclo- 



