300 MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 



mass of large and small ganglion cells, witli the roots of the fibers to which they give origin. The 

 cellnlar mass belonging to each lobe is seen to be sepaiated by a median space free from ganglion 

 cells. (This large mass of cells, forming the bulk of each lobe, does not appear to be represented 

 by Viallanes either in his figures of actual sections or of his wax model, and they form much larger 

 and more bulky masses than he describes or figures.) In the section represented by Fig. 2, the 

 roots of the median-eye nerves (m. eye n. r.) are seen on their outer aspect to be fused with the 

 proximal end of the lateral-eye lobes. 



Fig. 2a represents the bundle of twisted fibers forming the " external chiasma" of Viallanes; 

 these fibers appear to end in irregular pear-shaped masses of punctured substance, and beyond 

 them is the zone of internal chromatic cells {ch. c. I.). In PI. VI, Fig. 1, at/' is seen on each side 

 a fascia arising from the large ganglion cells in the lower part of the median-eye lobe, passing 

 back into the cerebral lobes, and behind is a second fascia (/"). 



VII. The Cerebral Ganglia or Lobes. 



(Plates III, IV, V, 20,21, VI, VII, VIII.) 



The third and much the largest pair of lobes are the cerebral ganglia. They comprise the bulk 

 of the brain when we take into account the masses of small ganglion cells constituting the " corjys 

 pedoncule^^ of Viallanes. The central fibrillar or white portion of these ganglia are very irregular in 

 outline compared with the massive, more or less sjiherical cerebral lobes of the Arachnida; and 

 the cortical masses of small ganglion cells which enclose and also ramify through the cerebral 

 white mass are very different in disposition from the ganglionic cortex of the Arachnidan brain, 

 and they are enormously developed in the brain of Limulus. 



The white fibrillar portion of the cerebral lobes are arranged in two masses, one on each side 

 of the median line of the brain. These lobes are very irregular m outline, slender, and apparently 

 shrunken. They are narrow and thin, sending oft" irregular lateral lobules.* The cerebral lobes 

 are connected posteriorly by a thick cerebral commissure (c. com.). 



This commissure, as numerous sections show (Pis. II, Figs. 8, 9, 10; III, Figs. 11-13), consists 

 of fibrillie originating from the thin crescent-shaped central dense mass of small ganglion (chro- 

 matic) cells (c. com. cortex) near the posterior margin of the brain, and which overlies the median 

 portion of the origin of the great commissures leading to the oesophageal ring. 



The central or larger mass of the cerebral lobes does not pass much below the midcUe of the 

 brain. It gives off the pairs of upper and lower tegumental nerves, and the jjosterior lobules, one 

 on each side, serve as the origin of the great commissures connecting the biain with the a'sophag- 

 eal ring, which latter is originally formed by the coalescence of the neuromeres of the postoral 

 cephalothoracic appendage-bearing segments. 



In order to clearly bring out the relations of the cerebral lobes I will first describe the horizon- 

 tal sections of the young Limulus when about 2 inches in length, exclusive of the caudal spine. 



In a series of 43 sections the uppermost sections of the brain do not involve the fibrous matter 

 of the cerebral lobes. The first fibrous portion encountered in cutting sections from above down- 

 wards are the two subparallel curved lateral-eye strands, PL VII, Fig. 1, lat.-eye., n. r., which send 

 fibers to the anterior horn or process of the median-eye lobes. Between these two strands is a 

 thick, horseshoe shaped bundle of fibers, the incomplete circle opening anteriorly near the front 

 of the brain, and inclosing two large groups of ganglion cells. These are the roots or origin of the 

 median-eye nerves. 



In the second section below the one just described (PL VII, Fig. 2) the knife passes through the 

 lower part of the median-eye lobes and through the upper part of the cerebral lobes, but involving 

 the larger central mass of the lobes. The oesophageal commissures are seen to be large and thick ; 

 the commissures are reinforced by the fibers sent oft' from the small gi'oup of large ganglion cells 

 (I g &") (those on the other side, not seen in this section). There is also a gTOup of large gan- 

 glion cells (I g c') on each, side of the base of the brain. 



In or near the central part of the cerebral lobes, PI. VI, Fig. 1, in the fourth and fifth sections 

 below the one just described, the mode of composition of the fibrous portion of the lobes is seen. 



* These lateral lobules are the second and third masses of the lateral-eye lobes, according to Viallanes. 



