(1) A central region containing the commissural part, and continuous below with the ganglia of the cheliceræ.
(2) A dorsal region formed of two hemispherical lobes.
(3) A ventral anterior region.
The central region contains in its interior the commissural portion, forming a punctiform, rounded mass in each ganglion. A transverse commissure connects the two (vide fig. 20b).
The dorsal hemispherical lobes are derived from the part which, at the earlier stage, contained the semicircular grooves. When the supra-œsophageal ganglia become separated from the epidermis the cells lining these grooves become constricted off with them, and form part of these ganglia. Two cavities are thus formed in this part of the supra-œsophageal ganglia. These cavities become, for the most part, obliterated, but persist at the outer side of the hemispherical lobes (figs. 20a and 21).
The ventral lobe of the brain is a large mass shewn in longitudinal section in fig. 21. It lies immediately in front of and almost in contact with the ganglia of the cheliceræ.
The two hemispherical lobes agree in position with the fungiform body (pilzhutförmige Körpern), which has attracted so much the attention of anatomists, in the supra-œsophageal ganglia of Insects and Crustacea; but till the adult brain of Spiders has been more fully studied it is not possible to state whether the hemispherical lobes become fungiform bodies.
Hatschek[473] has described a special epiblastic invagination in the supra-œsophageal ganglion of Bombyx, which is probably identical with the semicircular groove of Spiders and Scorpions, but in the figure he gives the groove does not resemble that in the Arachnida. A similar groove is found in Peripatus, and there forms, as I have found, a large part of the supra-œsophageal ganglia. It is figured by Moseley, Phil. Trans., Vol. CLXIV. pl. lxxv, fig. 9.
The stomodæum is considerably larger than in the last stage, and is lined by a cuticle; it is a blind tube, the blind end of which is the suctorial pouch of the adult. To this pouch are attached the vertical dorsal, and two lateral muscles spoken of above.
The proctodæum[TN15] (pr.) has also grown in length, and the two Malpighian vessels which grow out from its blind extremity (fig. 20e, mp.g.) have become quite distinct. The part now formed is the rectum of the adult. The proctodæum is surrounded by a great mass of splanchnic mesoblast. The mesenteron has as yet hardly commenced to be developed. There is, however, a short tube close to the proctodæum (fig. 20e, mes), which would seem to be the commencement of it. It ends blindly on the side adjoining the rectum, but is open anteriorly towards the yolk, and there can be very little doubt that it owes its origin to cells derived from the yolk. On its outer surface is a layer of mesoblast.