OLFACTORY ORGAN OF LIMULUS. 1 63 



the white pigment, of the refractive visual rods or rhabdoms, and of their primi- 

 tive grouping into ommatidia. 



In young LimuH, the roots of the lateral olfactories become less compact, and 

 as they were seldom seen in methylene blue preparations, it was very difficult to 

 follow them. They appear to shift their point of attachment from the second 

 optic ganglion, toward the inner face of the olfactory lobes, near the tract uniting 

 the median and lateral eye centers (Fig. 51, oc.tr.) Whether they passed through 

 this tract to the olfactory lobes could not be determined. In a few cases (methyl- 

 ene blue) a small strand of fibers was seen to leave the main root and pass 

 mesially toward the horns of the olfactory lobes. (Fig. 51, z.) 



Median Olfactory Nerve. — When the united olfactory placodes move for- 

 ward away from the brain, a new outgrowth from the hemispheres and ol- 

 factory lobes appears which follows the placodes forward, or is drawn out by 

 them, to form the median olfactory nerve. (Figs. 38, 39, 41, 48, 66, ol.m.n.) It 

 consists of large globular masses of minute ganglion cells, each lobule con- 

 taining a central core of medullary substance, similar to that in the 

 hemispheres. 



In young Limuli (2 to 3 inches long) there are four distinct roots to the median 

 nerve, two haemal ones continuous with the horns of the olfactory lobes, and two 

 neural ones, continuous with the anterior median lobes of the cerebral hemispheres. 

 (Fig. 48.) Each root contains a medullary core of neuropile surrounded by a 

 cortex of "granule cells," the cortex and neuropile passing without perceptible 

 change into the cortex and the neuropile of the cerebral hemispheres and the ol- 

 factory lobes. (Figs. 48, 51.) 



In the adult, the two haemal stalks disappear, while the two neural ones unite 

 and shift their attachment in a neuro-posterior direction, so that they are ultimately 

 widely separated from the apices of the olfactory lobes. 



In larvae about two inches long the distal ends of the three olfactory nerves 

 form a rich plexus of nerves terminating in a small patch of ectoderm that may 

 then be recognised as the definitive olfactory organ. 



Summary. — The lateral olfactory nerves, then, are characterized as follows: 

 The "ganglion cells" are large and pear-shaped, and arranged in small ommatidia- 

 like clusters. Granule cells and neuropile are never present. The fibers are 

 coarse tubes, with distinct sheaths. The nerves terminate in the lateral portion 

 of the olfactory organ and in the surrounding integument. The ganglion cells of 

 the lateral olfactories are the metamorphosed visual cells of the initial olfactory 

 organ. 



The median olfactory nerve represents a later, or secondary, outgrowth of the 

 hemispheres and of the olfactory lobes. Its ganglion consists of lobular masses of 

 granule cells and neuropile, and never contains large cells of a sensory nature. 

 Its end branches are bundles of naked fibers, or at least they have no visible 

 sheath. They terminate in the central region of the olfactory organ. 



