2 6 OUTLINE OF THE ARACHNID THEORY. 



there are frequently developed adhesive discs (phyllopods), or root-like out- 

 growths (cirripeds, copepods) that serve as organs of attachment, or for the ab- 

 sorption of nutriment. 



The cephalic navel of arthropods may be regarded as one of the inevitable 

 products of apical growth on a spherical yolk surface, just as the belly navel of 

 vertebrates is a product of the peculiar method of closing up the haemal surface. 

 The center, around which the converging lips of the cephalic navel are formed, 

 is the degenerating area of haemal blastoderm, often called the dorsal organ. 



The Closure of the Old Mouth or Neostoma. 



In the arachnids, there is a progressive enlargement and fusion of the an- 

 terior cephalic neuromeres, that gradually leads toward the narrowing of the 

 passageway for the stomodaeum, and ultimately to the closing of the mouth. The 

 backward growth of the rostrum and the transfer of the optic ganglia to the 

 region overlying the mouth, due apparently to remote, but persistent and cumu- 

 lative causes, are contributory factors in bringing about this result. 



These conditions at first lead to a profound modification of the mode of life, 

 making a liquid, or finely divided diet a necessity, and ultimately to the utilization 

 of the cephalic navel as a new entrance to the alimentary canal. 



Conclusion. 



In the arachnids, the body is built up by successive generations of new groups 

 of metameres, or tagmata, at definite historic periods in the evolution of the phylum. 



The process of cephalizing the anterior regions of the body consists in the 

 gradual and extensive elimination of motor elements and the establishment of a 

 definite sequence of functions and organs, according to an inherently necessary 

 order. 



The first five tagmata embrace the first sixteen metameres and lay the founda- 

 tions for the head in vertebrates. Each tagma is characterized by a special number 

 of metameres, by peculiarities in the number and structure of its neuromeres, 

 sense organs, ganglia, nerves, mesoderm and endo-skeleton, and by their sequence 

 and mode of growth, that are in essential agreement with those in the corre- 

 sponding divisions of the vertebrate head. 



The arachnid body consists of metameres added to the primitive head, which 

 represents the remnants of the coelenterate body. The greater part of the arachnid 

 body and its primitive head forms the vertebrate head. Nearly all the vertebrate 

 body consists of a new generation of metameres, not represented in arachnids. 



The conditions created by apical growth, by cephalization, and by the increase 

 in the volume of the yolk sphere, lead to the closure of the old mouth, and to the 

 formation of a new one on the haemal surface, the primitive dorsal organ forming 

 the starting point for the cephalic navel, that ultimately becomes the new mouth. 



