No. I.] STUDIES ON LIMULUS. 1 35 



side the bases of the appendages, and extend far back upon the 

 abdomen. 



The pre-oral cross-commissure {a.c), which belongs to this 

 neuromere, is more neural in position than the post-oral ones, 

 and is also characterized by the fact that it gives off three 

 nerves {la.n.) to the rostrum, or upper lip. 



In addition to these peculiarities the cheliceral neuromere 

 bears upon its inner side a pair of stomodaeal ganglia and a 

 pair of stomodaeal nerves {st.ii.) which supply the oesophagus 

 and proventriculus. 



(3) The Hind-Brain. — The hind-brain consists of five thoracic 

 neuromeres, the second to the sixth inclusive. Each neuromere 

 (Text-fig. 9) consists of a pair of ganglia united by cross-com- 

 missures, and two pairs of nerves, a neural pair («.«.) supplying 

 the appendages, and a haemal pair [h.n.) supplying internal or- 

 gans and the lateral expansions of the carapace. The mandibular 

 nerves {m.n.) arise from the bases of the neural nerves upon the 

 neural side, and the ento-coxal nerves, three in number {a.e.n.y 

 p.e.n., and m.e.n.), upon the haemal side of the same nerves. 



A pair of nerves (Text-fig. 13 ; PI. X, Figs. 1 1 and 12, i.n.^), 

 which resemble in their distribution the intestinal nerves, arise 

 from the haemal sides of the bases of the haemal nerves {k.n.^) 

 of the second thoracic neuromere. In the third, fourth, and 

 fifth neuromeres the intestinal branches are absent ; in the 

 sixth they are again present (i.n.^), but at some distance from the 

 bases of the haemal nerves {Ji.n.^). 



In the sixth neuromere a cardiac branch (Text-figs. 3 and 9 ; 

 Pis. VI-IX, Figs. 1-3 and 5, s.c.n.^) arises from the haemal 

 nerve still farther out than the origin of the intestinal nerve. 

 A small nerve (Pis. VI and VII, Figs, i and 2, a:.), which could 

 not be traced out, was found arising from the haemal nerve 

 (h.n.^) of the fifth neuromere, and apparently corresponding to 

 the cardiac nerves of other segments. 



(4) The Accessory Brain. — This contains two neuromeres, 

 the chilarial and opercular neuromeres, which are fused with 

 each other and form the posterior side of the circum-oeso- 

 phageal collar. Each contains the usual number of elements 

 of the typical neuromere. 



