12 



OUTLINE OF THE ARACHNID THEORY. 



of the arachnid brain is modified by the closure of the old mouth, and by the loca- 

 tion of the optic ganglia over the diencephalic and mesencephalic neuromeres, 

 instead of over the prosencephalic ones, to which they really belong. (Figs. 46, 

 47, 57 and 58.) 



The Mesoderm. (Fig. 138, A and B). The procephalic mesoderm is scanty 

 and in the early embryonic stages forms a single, thin-walled ccelomic chamber. 

 In the dicephalon and mesocephalon, six pairs of ccelomic chambers are formed, 

 constituting true somites, or head cavities; but segmented lateral plates are con- 



oc. 



ol.o. pa.ey. Olfactory. 

 Pro.C. ---._ >^T^\ Coord 



FIG. 5. Diagrams showing the probable relations between the subdivisions of the head and trunk, and the location 

 of the principal organs in an insect, merostome and ostracoderm (Bothriolepis) seen from the neural side. 



spicuously absent. In the metacephalon and branchiocephalon, distinct somites 

 and lateral plates are developed in each metamere. 



The Middlecord, or lemmatochord (notochord of vertebrates), extends through 

 the posterior sections of the head. In the older stages it may terminate in an 

 enlargement in the mesocephalon, but it never extends beyond the dicephalon, 

 ending abruptly just behind the stomodaeum (infundibulum). 



Let us examine these subdivisions of the future head more carefully. 



i. The Procephalon. 



The procephalon is the primitive head. In the adult arachnids, it is, exter- 

 nally, an irregular, ill defined area of ectoderm within which lie the rostrum, and 

 the primitive visual and olfactory organs. (Figs. 149-155, p.c.) In the early 

 embryonic stages, it is represented by the procephalic lobes, from which the fore- 



