THE ANCESTRY OF THE VERTEBRATES 523 



numerous other parts are more or less comparable. For ex- 

 ample, the eyes of the Merostomata consisted of a pair of 

 widely divergent lateral eyes, and a pair of closely ap- 

 proximated median eyes, a peculiarity seen in the present- 

 day Limulus; in the vertebrates there are the same widely 

 divergent lateral eyes, and the median pair is well repre- 

 sented by the pineal eye, in the development of which there are 

 many suggestions of its having been double at an earlier 

 period. In Limulus the central nerve-complex is protected by 



FIG. 142. Comparison of heart and gill arches of (a) Arachnoid, and 

 <b) Vertebrate. [After PATTEN.] 



.an internal skeletal piece, variously called " sternum " and 

 " endocranium," which in general form and still more in its re- 

 lationships to other parts resembles the primordial skull of 

 vertebrates. The arterial system shows in each case a tubular, 

 somewhat contorted heart, an arterial trunk proceeding anteri- 

 orly from this, and dividing into pairs of arterial arches, from 

 which, after sending a branch to the head, the blood is re- 

 collected into two lateral channels, in the one case a sinus, 



