No. I.] STUDIES ON LIMULUS. 99 



adult is against such a view of its origin. In another paper 

 we shall describe the central terminations of this nerve and 

 we will then discuss its probable relations to corresponding 

 cranial nerves in vertebrates. 



I had hoped to find some points of comparison between 

 vertebrates and Limulus in the mode of union of the cardiac 

 nerves with the central nervous system, especially in the rela- 

 tions of the cardiac branches to those cranial nerves arising 

 from what I have called the vagus region of the brain. These 

 inquiries at once led to the discovery of the three great lon- 

 gitudinal cardiac nerves, the nerve plexuses uniting them with 

 one another, and the segmentally arranged nerves by which 

 the cardiac plexus is united with the central nervous system. 

 When we came to study these nerves more carefully we found 

 that we had obtained data showing in Limulus an approach 

 toward the well-known conditions in vertebrates. This resem- 

 blance is shown : (i) by the absence of cardiac nerves in front 

 of the sixth thoracic neuromere ; (2) by the increased size, and 

 (3) by the fusion of the branches arising from the nerves of the 

 vagus region. In the transition from invertebrates to verte- 

 brates, we may assume that as the elongated arachnid heart was 

 forced forward into the head region it gradually lost its connec- 

 tion with the spinal nerves and retained its union with the 

 nerves of the vagus region only. In Limulus the fusion with 

 each other of at least two cardiac nerves in the vagus region, 

 and their increased size, indicate the adaptability of these 

 nerves for this purpose. 



The main sympathetic system of Limulus also presents strik- 

 ing resemblances to the condition found in primitive vertebrates. 

 Instead of the single trunk described by Milne-Edwards, we find 

 two longitudinal trunks, suggesting the condition found in 

 some Amphibia. One trunk is mainly related to the cardiac 

 nerves (PI. IX, Fig. 6), the other is deeper and nearer the 

 median line and more intimately related to the intestine 

 (PI. VIII, Fig. 4). Both sets of nerves form anastomosing 

 plexuses. They are united by regularly arranged rami com- 

 municantes with the haemal nerves, give off segmental 

 branches to the heart and intestine, and in addition supply 



