124 GENERAL AND SPECIAL CUTANEOUS SENSE ORGANS. 



In the Arthrodira (Fig. 89, C), the most important advance is in the appear- 

 ance of a distinct supra-orbital line, s.o.L, extending forward between the now 

 widely separate lateral eyes; and a distinct hyomandibular line h.ml. extending 

 toward, and probably onto, the greatly enlarged mandibles. In the arthrodira 

 the neuromasts apparently never form a series of separate dots and dashes, but 

 lie in continuous grooves of varying depth. 



In true vertebrates, no important changes or new conditions arise. The sev- 

 eral lines may be deeply infolded and joined at their proximal ends to form a 

 united series of canals, with the sense organs located in them at regular intervals, 

 suggesting the interrupted surface grooves of the ostracoderms. (Fig. 89, 

 B and D.) 



