THE ORIGIN AND EVOLUTION OF THE VERTEBRATES 79 



there are many investigators who seek the ancestor of vertebrates in 

 some worm-like form, there are few who wish to definitely assert 

 that this ancestor was an annelid." 



The writer sees nothing in the annelid theory seriously out of har- 

 mony with the Amphioxus theory, for Amphioxus may have been 

 derived from some early metameric, coelomate type that would be as 

 much like an annelid as anything else. It is much more likely, how- 

 ever, that the resemblances between annelids and vertebrates are 

 merely the necessary similarities that result from the fact that they 

 are constructed upon the same fundamental lines. Given two groups 

 having in common the axis of polarity, the dorso-ventral, the bilateral 

 axes, together with a metameric arrangement of organs, and the 

 expressions in structural differentiation must of necessity be fun- 

 damentally similar. Their differences are more puzzling than their 

 resemblances, and therein lies the real problem. 



THE THEORY OF ARTHROPOD ANCESTRY OF THE VERTEBRATES 



This theory is presented compactly by Lull, as follows: 



"In addition to the annelid theory, recent authorities have 

 tried to prove vertebrate descent from the Arthropoda, especially 

 from the more primitive arachnoids such as today are represented 

 by the scorpion and the horseshoe crab (Limulus) and formerly by 

 the extinct Merostomata. By this hypothesis we must set aside 

 as primitive such forms as Amphioxus and the cyclostomes and 

 start with the highly specialized ostracoderms which lived in Or- 

 dovician and Devonian times and thus were contemporaneous 

 with and in general appearance and probable habits quite similar 

 to the Merostomata. The soft parts of the Merostomata are of 

 course unknown, but it is reasonable to suppose that they were 

 not unlike those of the related scorpions and Limulus, and, as 

 Patten has shown, especially in the brain and cranial nerves of 

 vertebrates and the fused cephalothoracic ganglionic mass found 

 in such arachnoids, there are many points of resemblance (Figs. 

 43 and 44). Then, too, the sense organs, especially the eyes, are 

 more or less comparable, and there is in Limulus an internal skele- 

 tal piece known as the ' endocranium ' or sternum which serves 

 to protect the central nerve complex, and which in general form 

 and in its relation to other parts resembles the primordial 

 vertebrate skull. Similarities also exist between the heart and 



