324 



COLLEGE ZOOLOGY 



time in their lives. Although this notochord 

 persists in some of the lower vertebrates, it 

 becomes modified by an investment of car- 

 tilage (Fig. 207) which becomes segmented 

 and constitutes the vertebral column. In the 



higher vertebrates the vertebral column is 

 made up of a series of bodies called verte- 

 brae, and the notochord disappears before 

 the adult stage is reached. Seven classes of 

 vertebrates are recognized. 



Nerve cord 



Notochord sheaths 



Dorsal aorta 



-Notochord 

 — Cartilage 



Figure 207. Notochord of a young dogfish. Cross section shows nerve cord and sheaths of 

 notochord. 



Plan of structure 



The vertebrates resemble the other 

 chordates in their metamerism and bilateral 

 symmetry, in the possession of a coelom, a 

 notochord, and gill slits at some stage in 

 their existence, and a dorsal nerve tube. 

 They differ from other chordates and resem- 

 ble one another in the possession of cartilag- 

 inous or bony vertebrae, usually two pairs 

 of appendages, an internal and jointed 

 skeleton, a ventrally situated heart with at 

 least two chambers, and red corpuscles in 

 the blood. 



The body of a vertebrate may be divided 

 into a head, usually a neck, and a trunk. 

 In many species there is a posterior exten- 

 sion, the tail. Two pairs of lateral append- 

 ages are generally present, the thoracic (pec- 

 toral fins, forelimbs, wings, or arms) and 

 the pelvic (pelvic fins, hindlimbs). The 

 limbs support the body, serve in locomotion, 

 and usually have other special functions. 



The plan of structure of a vertebrate can 

 be presented most clearly with diagrams 

 showing longitudinal and cross sections 



through the body (Figs. 208 and 213). As 

 in the amphioxus, the nerve cord is dorsal 

 but extends in front of the end of the noto- 

 chord and enlarges into a brain. The noto- 

 chord becomes invested by the vertebrae. 

 The coelom is large. The digestive canal 

 forms a more or less convoluted tube which 

 lies in the body cavity. The liver, pancreas, 

 and spleen are situated near the digestive 

 canal. In the anterior trunk region are the 

 lungs and heart. The kidneys and gonads 

 lie above the digestive canal. 



Classes of Vertebrata 



The principal classes of vertebrates having 

 living representatives are: 



Class 1. Agnatha (L. a, without; Gr. gnathos, 

 jaws). Lampreys and hags (Fig. 

 240). Cold-blooded (poikilother- 

 mous), fishlike vertebrates without 

 scales, jaws, or lateral fins. 



Class 2. Chondrichthyes (Elasmobranchii) 

 (Gr. chondros, cartilage; ichthys, 

 fish). Sharks, rays, skates, and chi- 



