THE OLFACTORY LOBES IN LIMULUS. 165 



cells, however, remain very small, and of uniform size from one end of the lobe 

 to the other; toward the center of the lobes, the cells gradually increase in size. 

 (Figs. 51 and 66.) 



The small marginal cells send their neurites into two sharply denned bands 

 of very dense neuropile extending round the lobes. (Fig. 48, ol.np.) In the an- 

 terior horns, these bands become smaller, and unite to form a single band. The 

 latter extends to the apex of the horns, and is continued into the neuropile axis of 

 the median nerve. (Fig. 51.) 



In methylene blue, either one or both of the bands often stand out very clearly, 

 with only a single regular row of nerve cells visible over each band. The dendrites 

 of these cells are very minute, show a longitudinal trend, and are confined to their 

 respective bands. See the posterior median part of the olfactory lobes in Fig. 51. 



On the inner face of the deeper band (Fig. 48, ol.c 1 ' 4 ) are two small bundles of 

 longitudinal fibers, derived in part from medium sized cells on the inner margin of 

 the lobes. (Fig. 51, ol.c 2 .) These bundles are continuous with the tracts arising 

 from the median eye centers. One or two bundles of heavier fibers lie below and 

 concentric with the ones just described. They arise from the cells of the fourth 

 optic lobe, op.f 4 . On reaching the opposite side, they turn outward and back- 

 ward, and join the main longitudinal, haemo-lateral tracts, c.op.f*. 



Of the larger central cells of the olfactory lobes, we may recognize special 

 clusters of medium size cells sending neurites into the neuropile terminals of the 

 two pairs of median eye nerves, and hence to the circular bundles and to the tract 

 connecting them with the lateral eyes. (Fig. 51, ey.r*.) 



Farther back is a large cluster of cells, generally very conspicuous, sending 

 richly branched neurites outward and backward, underneath (on the neural side), 

 the main marginal bands of the olfactory lobes, into the longitudinal haemo-lateral 

 tracts of the same side, or in wide curves to the same tracts of the opposite side. 

 (Fig. 51, olc\) 



Finally in the posterior bend of the olfactory lobes there are some very large 

 deep lying median cells that send their enormous branching neurites backward 

 into the median neuropile mass of the cheliceral ganglion and hence right and left 

 along the median haemal side of each crus ol. c 5 . 



In the older crabs, the horns of the olfactory lobes gradually withdraw in a 

 posterior haemal direction and finally lose their connection with the median ol- 

 factory nerve root. Thus the main center of the olfactory lobes becomes relatively 

 isolated, unless, as it appears probable, the lateral nerves ultimately establish a 

 connection with them, through the median and lateral eye tracts. 



III. THE OLFACTORY ORGANS IN PHYLLOPODS. FRONTAL ORGANS. 



The olfactory organ of Limulus undoubtedly represents a highly specialized 

 condition of the characteristic "frontal sense organs" of the phyllopods. Each 

 resembles the other in location, innervation, origin, and histological structure. 



