EOEPHOLOGY OF BPAIN AND SENSE OEGANS OF LIMULUS. 23 



The lateral olfactory nerves arise apparently from the 

 anterior part of the brain, but in sections one can follow their 

 roots on to the ventral surface into the optic ganglia (fig. 49). 

 In the adult the proximal ends of the nerves consist of coarse, 

 transparent nerve-tubes, while their distal extremities contain 

 many large ganglion-cells, which form an elongated swelling 

 at the tip of the nerve : the latter terminates abruptly just 

 beneath the cuticle on the lateral edge of the olfactory organ 

 (fig. 49). The lateral olfactory nerve is accompanied by a large 

 blood-vessel that divides into numerous branches, supplying the 

 tissues in front of the olfactory organ (fig. 18, bl. v.) ; small 

 nerve-filaments accompany some of these blood-vessels, and 

 probably supply the ectoderm of the same region. 



Several larger nerve branches leave the median border of 

 the lateral nerves a little distance back of the olfactory organ, 

 and mingle with the plexus formed by the median nerve 

 (fig. 18). Some of these branches terminate in small, rounded, 

 ommatidia-like clusters of large cells, which contain irregular 

 refractive plates like those seen on the borders of the young 

 lateral eyes. The lateral olfactory ganglion contains a great 

 many of these clusters of cells. A fair idea of their appear- 

 ance may be had from fig. 20, which represents some of them 

 at the root of the lateral nerve in an immature specimen. 

 Some near the tip of the lateral nerve, in a still younger 

 specimen, are shown in fig. 22 [g. ol.n.). The lateral ganglion 

 and the isolated clusters of cells are the remnants of a 

 primitive sense organ ^ derived originally from the margin of 

 the brain, but which is now converted into these ganglion-like 

 cells. They are retinal cells that have lost their pigment, 

 and now have all the appearances of ganglion-cells, except 

 that they still retain their rods. In other words, we have 

 caught a sense organ in the act of being transformed into a 

 ganglion — the only case of this nature on record, so far as I 

 know. 



It is an extraordinary fact that the lateral nerve terminates 

 abruptly in this great mass of ganglion-cells, which are 



> See pp. 29-30. 



