No. I.] STUDIES ON LIMULUS. 145 



A blood vessel accompanies the stomodaeal nerve and com- 

 municates with the aortic arch at the side of the proventriculus, 

 and small vessels from the aortic arch accompany the branches 

 of the stomodaeal nerve over the proventriculus. 



(4) Rostral Nerves. — Milne- Edwards described two rostral or 

 labral nerves arising from the cerebral lobes. Patten found 

 three, a median and two lateral nerves (Pis. VII, VIII, and X, 

 Figs. 2, 3, II, and 12, l.a.n.), and correctly described them as 

 arising, not from the cerebral lobes, but from the pre-oral com- 

 missure. They innervate the rostrum or upper lip. 



c. Nerves from the Hind-Brain (Text-fig. 11). 



The second, third, fourth, fifth, and sixth thoracic neuromeres, 

 which make up the hind-brain, are so much alike that a descrip- 

 tion of one will, with a few modifications, suffice for all. 

 The third neuromere (Text-fig. 11) is most characteristic and 

 contains the usual elements, a pair of ganglia united by cross- 

 commissures, a pair of neural and a pair of haemal nerves. 



(i) Neural Nerves. — The neural nerves of the hind-brain 

 arise from ganglionated bases and radiate from the nerve collar 

 to the five pairs of appendages. Owing to the increasing dis- 

 tance from the brain to the base of the appendage, the basal 

 portions of the more posterior nerves are elongated, and the 

 entocoxal and mandibular nerves, which in the anterior neuro- 

 meres arise close to the brain, in the sixth neuromere arise 

 at a considerable distance from it (PI. X, Figs. 11 and 12). 



The typical neural nerve (Text-figs. 11, 12, and 8; PI. X, Figs. 

 II and 12, «.«.^-^) divides, soon after leaving the brain, into 

 three portions, a mandibular portion [jn.n.) supplying the gusta- 

 tory organs of the mandibles, a pedal portion {i.p.n. and e.p.n) 

 supplying the main portion of the appendage, and three ento- 

 coxal branches (^.«.) supplying the tergo-coxal muscles and the 

 sensory knobs of the coxopodite. 



(a) Mafidibidar Branches. — These nerves were first described 

 by Patten ('93). In the third neuromere there are three man- 

 dibular branches (Text-fig. 11, m.n) which arise close together 

 from the neural side of the nerve, not far from the brain. If 



