TAIL 



103 



This ventral lobe is invariably supported by single rigid skeletal 

 pieces, the haemal arches. Some traces of subdivision may be 

 found in Elasmobranchs, and distal nodules of cartilage may be 

 present in these and other fish ; but it is still an open question 





Fig. 04. 



A endoskeleton of the tail of Cotttts gobio, Ij. (After Lotz.) B, tail of Pachyeornvus heterurus, 



Ac showin" the last external trace of the heterocercal structure, a, bony sheath of tip of 

 notochord ■ as, scales covering the upturned tip of notochord ; d, dorsal radials(?) ; d.l, dorsal 

 lobe of hypochordal fin;/, fulcra; h, hypural bone; h.a, haemal arch; n.a, neural arch and 

 spine ; v.c, vertebral centrum ; v.l, ventral lobe of hypochordal tin. 



whether the radials have fused with the arches, are represented by 

 the nodules, or are altogether absent (Fig. 46). 



Doubtless the notochordal axis primitively extended in a 

 straight line to the extremity of the tail, as it still does in Cyclo- 

 stomes and Cephalochordates. The median fin fold probably 

 stretched continuously . long the dorsal edge to the tip of the tail, 

 and again ventrally from the tip to near the anus (p. 71). But 

 such a simple structure is rarely if ever found in either living or 



