302 MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 



tion of the cerebral lobes is uow smaller. One of the sheets of small ganglion cells is seen oil the 

 left side to throw off a sinuous bundle of fibers to aid in forming the oesophageal commissure of the 

 left side (fasc); and a little within is seen on the same side a group of large central ganglion cells, 



In the section represented by PL III, Fig. 13, the central masf.es of large ganglion cells, 

 directly under the median-eye lobes are seen to send their fibers outward from the median line of 

 the brain, and on the left side the sheets of small ganglion cells (sm. g. c.) are distinctly seen send- 

 ing delicate fibers into the commissure. In a few sections lower down (PI. IV, Fig. 16) the anterior 

 region of the white or fibrous portion of the lateral-eye lobes has diminished, while the section 

 l^asses through near tlie middle of the oesophageal commissures, this section also passing through 

 the middle of the entire brain, just at the lower edge of the pair of fi-onto-inferior tegumental 

 nerves (the nerves to the sensorial pit, of Viallanes). At gc. is seen a group of large ganglion cells, 

 sending a bundle of fibers to aid in making up the oesophageal commissures. 



Fig. 17 (compare the transverse sections, PI. XIII, Figs. 10, 11) represents a section through 

 the two groups of central and the lateral groups of cerebral ganglion cells; of the latter Fig. 18 

 is an enlarged view. On the right side is seen a portion of the stalk of the mushroom-body (>s<. 

 in. h.). In a few sections lower down the large ganglion cells are seen to be unipolar and to semi 

 their fibers into the oesophageal commissures, and the small ganglion or chromatic cells are also 

 seen sending their fibers into the commissure. Fig. 19 is an enlarged portion a few sections lower 

 down, showing the bundle of fibers arising from the group of lateral cerebral ganglion cells. 



Figs. 20 and 21 represent sections passing through the lower part of the cerebral lobes, where 

 the central group of large ganglion cells is larger; at I. gc. is an unusually large ganglion cell with 

 a very thick fiber passing from it into the commissure. 



In a few sections below (Pigs. 21, 22, 23) the large ganglion cells disappear, and also the com- 

 missures, the lower third of the brain passing below the level of the oesophageal commissures, and 

 the mass being filled with the sheets of chromatic ganglion cells. (See PI. XXXI). 



The longitudinal (sagittal) sections of the brain (PI. IX, X, Figs. 1-13, and PL XXXVI). — An 

 examination of the figures on Pis. IX and X will show how large a proportion of the brain is 

 occuiiied by the cortical masses of chromatic cells, which, in addition to their forming the sides of 

 the brain, occupy the lower half of the central portion. In PI. XX, Fig. 1, the cut through one 

 side of the brain passes through the outside of the base of the lateral-eye nerves, and also shaves 

 the nerve of the median eyes, but no large ganglion cells are involved, or the central fibrous sub- 

 stance of the cerebral lobes. In succeeding sections nearer the median line of the brain, as those 

 reijresented by PL IX, Figs. 2, 4,* and especially 3, the relations of the lateral-eye lobes to the 

 cerebral lobes are brought out, the two lobes being separated by a thick screen of chromatin-cell 

 plates or sheets, with their inclosed masses of fibrillar and myeloid substance belonging to the 

 mushroom bodies. 



The relations of the cortical masses of large ganglion cells of the median-eye and cerebral 

 lobes are to be seen in PL X, Pig. 11, the median-eye lobes being situated above and a Httle in 

 ti-ont of the cerebral large ganglion cells. In PL VIII, Figs. 5, 6, 7, three longitudinal sections of 

 the brain of the younger Limulus, which is larger and thinner than in very large specimens, are 

 represented, one (Fig. 5) showing the lateral-eye lobe and nerve, this section passing through the 

 oesophageal ring, while the other (Fig. G) illustrates a section passing througn the median-eye 

 lobe and nerve, and also the cerebral lobe. This section may be compared with that represented 

 by PL X, Fig. 11, of the large Limulus. 



The transverse sections of the brain (Pis. XI-XVI and XXXII). — In the large adult Limulus 

 the transverse sections show well the great amount of space taken up by the (more or less) cylin- 

 drical sheets of chromatic cells of the mushroom-bodies. In the anterior sections, involving the 

 origin of the lateral-eye nerves, the sections represented by PL XI, Figs. 1, 2, 3, and 5, pass ex- 

 clusively through the sheets of chromatic cells, which are thus seen to form not only the lower 

 portion but also the anterior region of the brain. At Fig. 3 the lateral-eye lobe on the right side 

 is involved. 



In the sections behind these toward the middle of the brain the anterior region of the cerebral 

 lobes is cut through, and the portions of the white or fibrous matter intersected are seen to be 

 confined, as in Figs. 4 and 5, to the upper third of the brain. 



* The flguro (4) referring to this figure has been inadvertently omitted by the lithographer. 



