i 3 2 THE APPEARANCE OF JAWS v. i- 



steering in the horizontal plane (p. 140). There are two dorsal fins, 

 which secure stability against rolling, and also assist in making possible 

 the vertical turning movements. 



The muscles for the production of these movements are a serial 

 metameric set, with longitudinal fibres, essentially like those of the 

 lamprey or amphioxus. The central axis is no longer simply a rod; 

 the notochord has become surrounded and partly replaced by a series 



df f s P .by Ld 



ac. 



bw. 



msv. pr b. 



Fig. 89. Diagram of the organization of a vertebrate. 



ac. wall of abdominal coelom ; b. body wall ; bd. basidorsal ; bv. basiventral ; bw. body wall ; dr. dorsal 



rib; i. intestine; iv. interventral; m. myocomma; ms. mesentery; msd. median dorsal septum; msv. 



ventral mesentery; nes. neural tube; ns. notochordal sheath; pr. ventral (pleural) rib; sp. neural 



spine; ts. horizontal septum. (From Goodrich.) 



of vertebrae (Fig. 89). These develop as two pairs of cartilaginous 

 nodules in each segment, the basidorsals and basiventrals behind, and 

 smaller elements, the interdorsals and interventrals, in the front. The 

 basiventrals, lying on either side of the notochord, form the centrum 

 of each vertebra, invading and almost interrupting the notochord, 

 which widens again, however, between the vertebrae. The vertebrae 

 are held together by ligaments, but are not articulated by complex 

 facets as they are in land animals. The basidorsals form neural arches 

 above the nerve-cord, and the interdorsals make intercalary arches. 

 The interventrals partly separate the centra. Attached to each basi- 

 ventral is a pair of transverse processes, which in the anterior region 

 bear short ribs and in the tail are fused in the midline to make the 

 haemal arches. 



The median and paired fins are supported by cartilaginous rods, 

 the radials, and their edges are further strengthened by special horny 



