PATTEN— COOPERATION AS A FACTOR IN EVOLUTION. 521 



If we compare the embryo of an arachnid, scorpion, or Limitlits, 

 with that of a primitive vertebrate, the basic similarity in their mode 

 of growth, the arrangement of their parts, and the natural transition 

 of one type into the other, is clearly apparent (Figs. 4a and 4b). 



In both cases, there are in their appropriate places, the medullary 

 plate, the chief sense organs, the neuromeres, somites, lateral plates, 

 heart, primitive streak, notochord, and germ cells ; there is also the 

 inevitable concrescence of the germ-wall as it spreads in an apico- 

 bilateral direction over the spherical surface of the Qgg, as well as 

 many other more detailed resemblances that we can not go into here. 



All these structural resemblances show that the chief points of 

 intake and discharge, the basic routes for the conveyance of com- 

 modities, and the sequence of way stations and terminals, that are so 

 characteristic of the vertebrate stock, are already well established 

 in the higher members of the arachnid stock. 



But there are some striking differences which we will briefly 

 consider, as they also illustrate the principles we have in mind. The 

 three most important ones are as f ollow^s : 



1. In the arachnids, the stomod?eum, or cesophagus, passes through 

 the floor of the medullary plate, while in vertebrates it lies in front 

 of it. 



2. In arthropods, there are several pairs of jaw-like appendages 

 which are located on the neural surface, and which, in chewing or 

 biting, move alternately in a right and left direction. In typical 

 vertebrates, the jaws consist of one or two unpaired arches, the 

 posterior arch being freely movable in an antero-posterior direction. 



3. The gill chambers of the vertebrates open into the alimentary 

 canal, while, in arachnids, they do not. 



In at least two of these cases, the differences are more apparent 

 than real, for the prevailing methods of growth in the arachnids, if 

 carried further, would ultimately lead to the conditions found in the 

 vertebrates. 



That is to say, the concentration of the cephalic neuromeres in 

 the head region of the arachnids has already narrowed the stomo- 

 dseal opening through the floor of the forebrain to a minute canal 

 barely large enough for the passage of fluids, thereby compelling the 

 great majority of arachnids to adopt a liquid diet. 



