THE DICEPHALON AND MESOCEPHALON. 15 



and by their union there with the haemal end of the first thoracic metameres. 



(Fig. 17-) 



In this way the original area of the procephalic ectoderm has been greatly 

 extended. In the adult Limulus, it is divided into two isolated parts: that which 

 has been carried onto the haemal surface of the carapace, and that which remains 

 on the neural surface. (Figs. 141-155.) The latter portion may be approxi- 

 mately defined as an elongated area, with the olfactory organ at its anterior end 

 and the apex of the rostrum at its posterior end; it is drawn out laterally by the 

 migration of the lateral eyes toward the posterior haemal surface. (Fig. 153, pr.c.) 



In the scorpion, there is a neural and haemal section of the procephalon, as in 

 Limulus. (Comp. Figs. 16, 17, 18,43.) The original neural surface of the em- 

 bryonic procephalon has been doubled over in the adult so that its anterior edge 

 lies on the haemal surface, directed backward instead of forward. (Figs. 17, 22.) 



FIG. 8. Primitive crustaceans (Cladocera). 

 A, Neural surface; B, haemal. 



FIG. 9. Same in side view and in median 

 section. 



It is important to bear these facts in mind, since where these changes have 

 taken place, the linear arrangement of the segmental sense organs appears to be 

 the reverse of what it is when the procephalon remains largely on the neural 

 surface, as it does in many phyllopods and vertebrates. (Figs. 7, 8, 9, 34.) 



2-3. The Dicephalon and the Mesocephalon. 



The dicephalon and the mesocephalon include the first six or seven post-oral 

 metameres, frequently spoken of as the thorax. It is generally divided into two 

 regions. The anterior one, the dicephalon, consists of two or three circum-oral 

 metameres whose appendages may be smaller than the others, and specially 

 modified to serve as leg-jaws for testing, holding, tearing, or crushing food, and 

 conveying it to the mouth. It includes the stomodaeum and the stomodaeal 

 ganglia, the latter being intimately associated with the gustatory and swallowing 

 reflexes. The posterior division, or mesocephalon, comprises three or four well- 

 developed metameres whose appendages serve for walking or swimming. 



