MORPHOLOGY OF BEAIN AND SENSE ORGANS OF LIMULUS. 61 



B. The Mid-brain. — The first post-oral neuromere, that 

 of the chelicerse, becomes intimately united with the cephalic 

 lobes, as in Insects and Scorpions. Its stomodseal nerves, with 

 one exception, are identical with those of Scorpions. There 

 is a large 'lateral stomodseal ganglion" on the median 

 margin of the cheliceral neuromere, from which two large 

 lateral stomodseal nerves extend inward the whole length 

 of the stomodseum (figs. 43, 46, 49,61,5^. n.). There are also 

 a small median and two lateral nerves extending outward and 

 backward along the oesophagus to the rostrum, comparable 

 with the paired and unpaired labral nerves of Insects and 

 Myriapods (figs. 47 and 48). The first post- oral commissure 

 differs from the others in that it is isolated from the rest, and 

 forms a long loop around the posterior under side of the ceso- 

 phagus. The other commissures extend straight across, and 

 are bound together by firm connective tissue. I have not 

 traced the ends of this commissure up to the lateral stomodseal 

 ganglion, but nevertheless it seems to me very probable that it 

 represents the "posterior pons stomodsei" of insects and 

 Myriapods. The commissure just in front of the mouth un- 

 questionably corresponds to the anterior pons stomodsei of 

 insects and Myriapods. 



I can find nothing in Limulus corresponding to the optic 

 ganglion of the second segment in Scorpions. This seems to 

 be due to the fact that the median eye-nerves in Limulus 

 have shifted their points of attachment from the neural surface 

 of the brain to the semicircular lobes on the opposite side. 



c. Later Modifications of the Brain. — After the stage 

 shown in fig. 59 the cephalic lobes rapidly change in aspect, 

 and one finds less and less resemblance between them and the 

 brain of other Arthropods; while, on the other hand, their resem- 

 blance to the fore-brain of Vertebrates becomes more apparent. 



Among the important changes that take place in the dis- 

 position of the parts is the shortening of the optic ganglia, 

 and the forward movement of their distal ends ; at the same 

 time they are drawn inward and backward till they lie in 



