4 OUTLINE OF THE ARACHNID THEORY. 



dicephalon, mesocephalon, metacephalon, and branchiocephalon. Each region 

 usually consists of a certain number of metameres, modified, or specialized, in a 

 very constant and definite manner in respect to its sense organs, nerves, and other 

 characters. In the higher arachnids they unite in various ways to form larger 

 aggregates, such as the cephalo-thoracic-branchial region. In the vertebrates 

 they have become still more compactly united to form the head, the subdivisions 

 of which still consist, as nearly as may be determined, of the same number of 

 metameres, modified in the same characteristic manner as the corresponding 

 subdivisions of the arthropod trunk and cephalothorax. (Figs, i, 3 and 5.) 



ol.o. . 

 prosen. 

 gust.o. 

 die.enc 

 mesen.c 



meten.c. 



ec.pa.e. 

 en.pa.e. 

 dor.o. 

 .I.e. 



FIG. i. Plan of a marine arachnid, based in part on Limulus. Designed to show the principal body regions 

 and their characteristic organs. A, Neural, or oral surface; B, haemal, or cardiac surface. 



B. Embryology. We shall show that arachnid and vertebrate embryos, 

 from the very beginning of their development, are fundamentally alike in structure 

 and mode of growth, and that this likeness is continued through successive, 

 parallel stages, up to a point where the arachnid stages cease; then the vertebrate 

 embryo, entering on its particular phases of development, carries them to com- 

 pletion. We shall show that the similarity between them consists: a. in the 

 origin of the germ layers; b. in the general form and segmentation of the neural 

 plate; its flexures, mode of enclosure, and the location of its principal parts; c. 



