MliMOiiib Oi-' lUE .NATIONAL ACADKMi OF aClli^^OES. 329 



KXI'LANATIOX OF I'LATKS. 



I'l., 1. FiKs. 1-7. Horizontal si>etion8 through tho brain of the adult Limulus passing from above downwards, involv- 

 ing till- liitoral oyo and niedian-oye lobes. In Fig. 5 the si'ction involves the iijiper i)art, of the 

 niedian-oye lobes (m. ei/c i.); the rulHe-like uiassfs of the mushroom bodies (m.li.) are seen in 

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

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



I'l.. II. Figs. la.~b. H-IO. Horizontal sections, continued, passing downwards through the same lobes. Fig. 7a, repre- 

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

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

 pare PI. VII, Fig. 2«,cft.c/.). Fig. 76, the external m- tliird division of the same lobe ("lame 

 gunglionnain" of Viallanes), still more enlarged showing the inner sheet of chromatic cells (c/i. cl.) 

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

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

 eye lobe; HI. 6., the upper masses of chromatic cells of the mushroom body; «;jp'. »i. nerve of 

 the lirst 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. vije n. 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 PI. vii, Fig. 1, m. ci/f n. r.) The 

 space bet ween tho roots of the median-eye nerves is seen to be filled with both largo 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. 



I'L. III. I'i^rs. 11-1.5. Horizontal sections, contiimed, passing through the niodian-eyo lobes. Fig. 11. This section 

 passes through the cerebral <'onimissure (c. I. cmti. ) with its crescent-shaped mass of chromatic gang- 

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

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



Pi.. IV. Figs, lfi-19. Passing through tho cerebral lobes. In Fig. 17 is seen a portion of the st.ilk of the mushroom 

 body («/. m. *.). Fig. 18. Enlarged view of a bundle of fibers on right side of the cerebral lobes, 

 arising from a group of chrora itic cells. Fig. 19. Enlarged view of a similar area in a section below. 



Pl. V. Figs. L'i>-22. Sections passing through the lower part 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 

 (/.«(.) and the anterior branch (a7i<. «(.). (Plate xxxi is photographed from a drawing of a 

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

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

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



I'l.. VI. Fig. 1. Horizontal section (No8. 43 and 44) through the brain ofa small Limulus, the body about 2 inches long, 

 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; ch. c, sheet of chromatic cells in the external 

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

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

 the ba.se of the brain, seen in old, large examples. 



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

 of fibers inclosing the large and small ganglion cells of the partially fused inedian-eyo ganglia or 

 lobes. Fig. 2. Tho entire lateral-eye lobe is seen on the right slile; l.g.o'"., a group of large 

 ganglion ci^Us sending a bundle of fibers backward into the (csopliageal commissure. Fig. 2o, 

 showing at c ch. the twist or " <'hiasma " (»ic) of the fibers connecting the second and third divi- 

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

 fibers forming the so-called chiasma. 



