MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 803 



As the knife passes through tlie posterior region of the lateral-eye lobes it also cuts through 

 the upper and anterior region of the median-eye lobes (Figs. 4 and 5). 



In the next section figui-ed, Fig. 0, the knife passed behind the lateral-eye lobes, involving 

 the median-eye lobes, and also the anterior region of the cerebral lobes whicli send lobules 

 into the lower part of the biain. The section, like all the others, being a little oblique, show the 

 white fibrous matter on the right side penetrating to the bottom of the brain. Fig. 6a is an 

 enlarged view of the central portion of the left median-eye lobe, showing the large ganglion cells, 

 which are seen to vary much in size; and the small ganglionic or chromatic cells forming the semi- 

 circular sheet or plate, and inclosing the fibrous matter as well as the myeloid or punctured sub- 

 stance. The inner layers of aprotoplasmic or chromatic cells (small ganglion cells) are here seen 

 to send their flbrilliB in toward the center of the mass inclosed by the cells. 



These small ganglion cells contain usually more than one nucleolus — usually two or three — and 

 I have counted in them from 1 to 6 nucleoli. The number of nucleoli in the large ganglion cells 

 varies froml to 4, but there are usually two. These cells are seen to be unipolar and to send their 

 nerve fibers directly across toward the groii^) of cells in the opposite lobe, though no distinct com- 

 missure is thus formed. 



In a few sections farther back the ganglion cells, both large and small, are arranged as in 

 PI. XI, Fig. 7, and PI. XII, Fig. 8 shows their arrangement and relations to the central and lateral 

 large ganglion cells in a section made still farther back, near the middle of the brain. 



In the section represented by PI. XII, Fig. 9, which passes through near the middle of the 

 brain, the two masses of large ganglion cells of the median-eye lobes liave coalesced, while the 

 cerebral ganglia are now larger, and the central and cortical groups of large ganglion cells are 

 larger than in Fig. 8. In the third section succeeding large ganglion cells appear on the left side. 

 In PL XIII, Fig. 10, the cortical groups of large ganglion cells disappear, and those of the central 

 groups become more numenms. This section involves the origin of the oesophageal ring, and now 

 the posterior end of the brain becomes flatter, wider, and not so deep. PI. XIII, Fig. 13, repre- 

 sents the posterior jiortion of the cerebral lobe, with the thick cerebral commissures. In the 

 second section still farther back the ganglion of the first pair of appendages, PI. XIII, Fig. 13, 

 is intersected, and in PI. XIV, Fig. 17, the knife passes through the esophageal ring just behind 

 the ganglia of the first pair of appendages. 



In the transverse sections of the brain of a small Limulus, about 2 inches long exclusive of the 

 caudal spine, the details brought out, in addition to those already pointed out, are the very distinct 

 basal portions of the white fibrous portion of the cerebral lobes. These are seen in PI. XIY, Fig. 

 18, to send an irregular branch on each side of the median line of the brain (»*) up to the top of the 

 brain, each branch sending three irregular lobes out to the side of the brain, besides a number of 

 smaller ones which are seen to be cut across, and enveloped by the thick sheets or cylinders of 

 small ganglionic or chromatic cells. (These branches of the cerebral lobes are what Viallanes re- 

 gards as the stalk of the mushroom or pedunculated bodies). In Fig. 18 the median fissure or si^ace 

 (m) between the sausage-like grc )ui)s of cliromatic cells is seen more distinctly than in the fully grown 

 individuals. In this section also the central groups of large ganglion cells {dgc) are seen above 

 the cerebral commissure (c. com), and are also arranged one group on each side of the median line 

 of the brain. 



PI. XV, Fig. 20, represents a section through the hindermost portion of the brain, the section 

 also passing through the ganglia of the first pair of appendages. It will also be seen here, as in 

 PI. VI, that the pair of ganglia of the first pair of appendages are quite distinct fi'om the brain, 

 the nerves to these appendages {1st app. ?*.) arising from the mass of large ganglion cells forming 

 a part of the oesophageal ring, 



VIII. Ebsults compared with those of other observers. 



As may be seen from the preceding descriptions and by reference to the plates, the results 

 of our examination of the brain of th§ adult Limuhis, made with every possible care and pains, and 

 based on many hundred sections, cut not only in a horizontal, but also longitudinal and transverse 

 direction, are the following : The brain of this merostome is composed of but three pairs of lobes, 



