522 HISTORY OF THE HUMAN BODY 



nishing material for the notochord. Attention has also been 

 called to the respiratory function of the anterior portion of the 

 intestinal canal in nemerteans. 



Aside from all hypotheses which have at their basis the con- 

 sideration of a worm-like ancestor may be briefly mentioned a 

 recent theory which finds the vertebrate ancestor among the 

 more primitive arachnoids, now represented by such animals, 

 as the scorpion and the horse-shoe crab (Limulns) and for- 

 merly exhibited by the extinct group of Merostomata. To ap- 

 preciate this one must at the outset dispose of the cyclostomes 

 and other low forms like Amphioxus as degenerate and with- 

 out special significance, and take as the starting point of verte- 

 brates such forms as the ganoids, or more especially the placo- 

 derms, which lived in Devonian times and were contemporaries 

 of certain aquatic arachnoids, allies of the horse-shoe crab. 



As the starting point in this theory there may be taken a 

 certain series of resemblances between the brain and cranial 

 nerves of vertebrates and the fused cephalo-thoracic ganglionic 

 mass found in such arachnoids as the scorpion and the horse- 

 shoe crab. In these forms this central mass is divisible into 

 three distinct portions, comparable to fore-, middle- and hind- 

 brains, with an accessory part corresponding to the medulla. 

 The number of neuromeres, or primary nerve somites of which 

 these parts are composed, i, e., 3-1-5 for the brain and 2 to 4 

 for the .medulla, also corresponds closely with the conclusion of 

 many specialists concerning the segmental values of those parts 

 of the head in vertebrates. A similarly suggestive resemblance 

 exists in the cranial nerves and the relations of the organs of 

 sense. 



Although the anatomy of the soft parts of the Merostomata 

 will never be known, they could not have been very different 

 from the condition found in Limulus and the scorpion, and it 

 may even be supposed that they and modern vertebrates have 

 developed in distinctly different directions from these as com- 

 mon ancestors, and that thus their condition may have been far 

 more like that of the vertebrates than is that of any of the 

 arachnoids now living. Aside from the nervous system, 



