MEMOIKS OF THE NATIONAL ACADEMY OF SCIENCES. 



291 



is separated from the (Bsophageal ring by a .slight but yet distiuct constriction seen both above and 

 beneath, the constriction on the upper or dorsal side passing just behind the lateral eye-lobes, viz., 

 those sending nerves to the large lateral compound eyes. It is broader at the base than it is thick, 

 and narrow in front, seen from above, so that it is about a fourth narrower in front (in front of 

 the origin of the lateral-eye nerves) than at the base or posterior end. 



The ventral or under side is quite regularly convex and fuller and rounder than above; the 

 convexity or fullness is greater in tlie center and drops or extends down considerably below the 

 ori"in of the nerves of the tirst pair of appendages, this being the portion below the central lobes 

 and which is tilled with the ruftlelike masses of small, ganglionic or chromatic cells. 



The brain of a Limulus about 10 inches long, exclusive of the caudal spine, is about 6 mm. 

 in diameter; it is conico-spheroidal, broad and flattened from above, and on the under side full 



uu 



fn.exje/it • 



Jr.i 



vx.i r. tcsr 



Fig. I. Brain and oesophageal ring of Limulus, seen from above; 6r., 

 brain; cc, commissures behind tlie cesophagus ; m. eye jt., median-eye 

 nerve ; lat. eye n., lateral-eye nerve : l-vi, nerves to the six pairs of cepha- 

 lothoracic appendages or legs ; v.c, ventral cord; I. teg., lower tegumental 

 nerve ; teg. n., other tegumental nerves. 



vc 



Fig. rt.- -Ventral view of the brain and tesophageal ring 

 of Limulus. m. fr. (e?., meilio-frontal tegumental nerve; 

 b. teg. 11., lower tegumental nerve or nerve to the sensorial 

 pit. Author del. 



and rounded; on the upper side is a broad, shallow, median furrow, indicating that it is double. 

 In the young Limulus, with a body about 2 inches long, exclusive of the caudal spine, the brain 

 is longer, narrower, and less spherical. 



Three pairs of nerves and a median unpaired one (the ocellar) arise ft-om the upper third of 

 the anterior face of the brain. The two lateral-eye nerves are the largest, arising very near the 

 upper side of the brain, one on each side of the median furrow, so that the second and third sec- 

 tions made by the microtome pass through them. Next below (from above downwards) is the 

 origin of the single nerve sent to the two ocelli. We have not traced this nerve as far as the 

 ocelli, but Milne-Edwards states that near the ocelli it divides into two branches. On each side 



