MORPHOLOGY OF BliAlN AND SENSE ORGANS OF LIMULUS. 51 



ease than in Acilius, especially the three invaginations of the 

 optic ganglion. 



In the first segment of Scorpions^ the ocelli, as well as the 

 distinction between brain-lobe and optic ganglion, have dis- 

 appeared. The invaginations, which in Acilius are separate, 

 here unite to form a great transverse furrow with thick walls and 

 small dark nuclei (fig. 58, s. I.) ; the whole lobe sinks below the 

 surface, and moving backward, lies underneath the second 

 segment, where it forms the semicircular lobes (l^organ 

 stratifie, I'organ peduncule), identical in almost every 

 particular with the semicircular lobes of Limulus and Spiders. 

 The second pair of invaginations are at first exactly like those 

 in insects; but the fold on the lateral margin of the invagina- 

 tion soon advances rapidly inwards and backwards, and uniting 

 in front with a similar fold in the first segment, and behind 

 with one in the third, gradually extends backwards, in a broad 

 amnion-like fold, over the whole of the cephalic lobes. During 

 this process the eyes on the second segment are gradually 

 infolded, so that they finally lie inverted on the middle 

 wall of the fold. They then unite with one another 

 over the median line, and growing forwards come to 

 lie in a common sac at the distal end of a short tube 

 (fig. 42). The eyes of the third segment are not involved in 

 the ganglionic invagination, consequently they remain upright 

 on the surface ectoderm, as in Acilius. Thus almost the 

 whole of the cephalic lobes are invagiuated, or, to speak more 

 accurately, they are enclosed by amnion-like folds to form a 

 primitive cerebral vesicle. The floor of the vesicle is 

 formed by the fore-brain or the whole of the cephalic lobes, 

 except that part containing the lateral eyes. The vesicle has 

 a thin roof or pallium, from which arises a median tubular 

 outgrowth, with its terminal pineal eye. There is nothing 

 approaching this in any other animals, except in the Verte- 

 brates, where this condition has long been regarded as typical 

 of the group. I am convinced that this fact alone, when 



^ See also my figures of the cephalic lobes of the Scorpion " On the Origin 

 of Vertebrates from Arachnids," vol. xxxi, part iii, of this Journal. 



