No. I.] STUDIES ON LIMULUS. 139 



passes forward neural to the oesophagus and a little to the right 

 of the median line ; turns haemally to the right of the proven- 

 triculus and passes to the median eye upon the haemal surface 

 of the carapace in the median line. 



According to Dr. Patten the median eye consists of two 

 ectoparietal and one endoparietal eye, and the endoparietal eye 

 is formed by the fusion of a pair of retinas. He finds that " the 

 distal end {of the median eye nerve) splits up into four branches, 

 two of which plunge directly into the median diverticulum or 

 endoparietal eye, and the other two pass to the paired retinas 

 of the ectoparietal eyes." 



(3) Lateral Eye Nerves. — The lateral eye nerves (Pis. VI 

 and VIII, Figs. 1-3, l.e.n.) arise from the large optic ganglia 

 upon the haemal side of the cerebral lobes, pass forward 

 median to the tergo-coxal muscles of the chelicerae, turn out- 

 ward around the anterior outer corners of the entocoxites of 

 the second pair of appendages, and then pass backward to the 

 lateral eyes. The distal extremity breaks up into two small 

 branches and a large one. The large one passes directly to the 

 retina of the lateral eye ; the two smaller ones pass farther 

 backward to a pigmented body beneath the retina. 



A large blood vessel encloses the lateral eye nerve for the 

 greater part of its course, and is continued beyond the lateral 

 eye where it anastomoses with the hepatic artery. 



Description of a Typical Neuromere. 



The nerves of all the neuromeres posterior to the fore-brain 

 conform more or less closely in their distribution to a common 

 plan, which it will be well to keep in mind as we take up the 

 successive neuromeres. 



(i) Abdominal Neuromere. — The most primitive, and hence 

 the most typical neuromeres are those in the abdominal 

 region, viz., those from the ninth to the thirteenth. In these 

 neuromeres (Text-fig. 8) we have a pair of ganglia {a.g.) 

 united by cross-commissures, and also partially fused in the 

 median line. Posteriorly, a pair of neural nerves arise, and, 

 anteriorly, a pair of haemal ones. The neural nerves {n.n.) 



