MORPHOLOGY OF BRAIN AND SENSE ORGANS OF LIMULUS, 59 



but the opening to the invagination is at right angles to the 

 longitudinal axis of the body instead of parallel with it, as in 

 the early stages of the Scorpion (fig. 59). But even in Scorpions 

 it is at right angles with the long axis of the body at a later 

 period. The sense organ on the anterior margin of this 

 invagination in Limulus, the primitive olfactory organ, must 

 be homologous with the lateral eyes of Scorpions. In the 

 anterior median part of the cephalic lobes another enlargement 

 of the furrow appears {s. I.) ; its walls are thicker and stain 

 deeper than elsewhere, forming two oblong, slightly thickened 

 lobes, very conspicuous in surface views, and having the same 

 appearance as the semicircular lobes of Scorpions and Spiders, 

 with which they are unquestionably homologous. This is 

 shown not only by their position and appearance in the early 

 stages, but by the fact that they develop into the same kind 

 of organs in the adult. Back of the semicircular lobes are 

 two poorly defined swellings {br.^ and br.^), that I regard 

 as the second and third segments of the fore-brain. From 

 the anterior lobes arise the cerebral hemispheres; the 

 posterior ones do not undergo any marked specialisation, they 

 are concealed by the subsequent backward growth of the 

 cerebral hemispheres, and seem to bear the same relation to 

 the rest of the fore-brain that the " tween-brain '' does to the 

 cerebral hemisphere in Vertebrates. 



About midway between the invagination of the optic ganglia 

 and that of the semicircular lobes appears a small pore, which 

 rapidly moves forward and inward till it lies in front of the 

 invagination of the cephalic lobes. The position of this pore 

 at first made me regard it as belonging to a segment in front 

 of the semicircular lobes, as I stated in my paper on the 

 " Origin of Vertebrates from Arachnids." Further study, 

 however, convinced me that it originates between the invagi- 

 nation of the semicircular lobes and that of the optic ganglion, 

 and consequently it belongs to the second segment of the 

 cephalic lobes. The pore leads into a short tube formed by 

 an invagination of the ectoderm. The tube, although easily 

 seen in sections, is not very clear-cut, and the lumen extends 



