MEMOllib UF THE i(AT10NAL ACADEMY OF aCIENUES. 329 



EXPLANATION OF PLATES. 



Pl. I. Figs. 1-7. Horizontal sections throngh the brain of tlie adult Liraulus passing from above downwards, involv- 

 ing the lateral eye and median-eye lobes. In Fig. 5 the section involves the upper part of the 

 median-eye lobes (»». eye I.); the ruffle-like masses of the mushroom bodies Cm. h.) are seen in 

 Fig. 1, on the right side, this and all the sections of this brain having been cut a little obliquely, 

 the brain not having been evenly placed in the object-holder. 



Pl. II. Figs. 7a, 76, 8-10. Horizontal sections, continued, passing downwards through the same lobes. Fig. 7a, repre- 

 sents the same section as Fig. 1, but enlarged ; eft. c7., the internal thin sheet of chromatic or small 

 ganglion-cells situated at the di.stal end of the external chiasma of the lateral-eye lobe. (Com- 

 pare Pl. VII, Fig. 2a,ch.cl.). Fig. 76, the external or third division of the same lobe {"lame 

 ganglionnaire" of Viallanes), still more enlarged showing the inner sheet of chromatic cells (eft. cl.) 

 and the fibrillie arising from them. Fig. 8, from a section lower down, passing through the median- 

 eyo lobes (m. eye I.) and involving on the right side the first and middle divisions of the lateral- 

 eye lobe; »)i. 6.,the upper masses of chromatic cells of the mushroom body; aj>2>'.n. neTve of 

 the first pair of appendages; m.eye «., median-eye nerves fused into one. Fig. 9, the same a 

 little lower down. Fig. 10, details of the median-eye lobes; m. eye ii. r., root or origin of the 

 median-eye nerves, seen to be separate from its fellow, and to arise from near the external aspect 

 of the lobes, while still more externally arise the lateral projection of the lobes (m. eye n. e.) the 

 fibers arising from a special mass of chromatic cells. (See also Pl. vii. Fig. 1, m.eye n.r.) The 

 space between the roots of the median-eye nerves is seen to be filled with both large and small 

 (chromatic) ganglion-cells ; c. com., the cerebral commissure, the fibers of which appear to arise 

 from the crescent-shaped mass of chromatic cells. 



Pl. III. Figs. 11-15. Horizontal sections, continued, passing through the median-eye lobes. Fig. 11. This section 

 passes through the cerebral commissure ( c. I. com. ) with its crescent-shaped mass of chromatic gang- 

 lion cells. Fig. 12. The oesophageal commissure connecting the cerebral commissure with the 

 oesophageal ring is seen on the right side of the section. 



Pl. IV. Figs. 16-19. Passing through the cerebral lobes. In Fig. 17 is seen a portion of the stalk of the mushroom 

 body {st. m. b.). Fig. 18. Enlarged view of a bundle of fibers on right side of the cerebral lobes, 

 arising from a grouj) of chrom itic cells. Fig. 19. Enlarged view of a similar area in a section below. 



Pl. V. Figs. 20-22. Sections passing through the lower ])art of the cerebral lobes and their "nucleogenous bodies," 

 forming the cortical portion of the numerous branches of the mushroom bodies. In Fig. 21 is 

 seen the basal portion of the stalk of the left mushroom body, with one of its lateral branches 

 (J. St.) and the anterior branch (anf. s/.). (Plate xxxi is photographed from a drawing of a 

 section lower down, below the cerebral lobes; the clear spaces being the fibrous substance of the 

 branches of the mushroom body which pass down to the base of the brain, and which, with 

 their ganglionic cortices, form the lower third of the brain.) 



Pl. VI. Fig. 1. Horizontal section (Nos. 43 and 44) through the brain of a small Limulus, the body about 2 inches loug, 

 exclusive of the caudal spine, and showing the entire lateral eye lobes; /'., a bundle of fibers 

 arising from the large ganglion cells in the median-eye lobes and passing backwards into the 

 cerebral lobes, meeting the fibers of its mate; /"., a second bundle of fibers meeting those of its 

 opposite bundle in the middle of the cerebral lobes ; eft. c., sheet of chromatic cells in the external 

 or third divisinn of the lateral-eye lobes. At this stage the ganglia of the first pair of append- 

 ages are seen to be entirely distinct from the brain proper, not showing the tendency' to fuse with 

 the base of the brain, seen in old, large examples. 



Pl. VII. Figs. 1-3. The same lower down. Fig. 1. Origin of the median-eye nerves; the hor.seshoe-shaped bundle 

 of fibers inclosing the large and small ganglion cells of the partially fused median-eye ganglia or 

 lobes. Fig. 2. The entire lateral-eye lobe is seen on the right side; l.y.c'"., a group of large 

 ganglion cells sending a bundle of fibers backward into the (esophageal commissure. Fig. 2o, 

 showing at e. eh. the twist or '• chiasma" (sic) of the fibers connecting the second and third divi- 

 sions of the lateral eye ganglia, and the sheet of chromatic cells which appear to give rise to the 

 fibers forming the so-called chiasma. 



