32 WILLIAM PATTEN. 



distinct botryoidal masses composed of irregular lobes of 

 small, densely packed, and deeply stained nuclei, each sur- 

 rounding a central mass of medullary substance (fig. 17). The 

 whole appearance of the lobes is very much like that of the 

 convoluted parts of the cerebral hemispheres (fig. 52). In 

 some cases there is only one large lobe, evidently formed by 

 the more or less complete fusion of two like those in fig. 17. 

 The stalk of the lobes in this stage is a solid column of small 

 nuclei, and passes without any perceptible change into the 

 cerebral hemispheres. The fusion is so complete that imme- 

 diately after reaching the brain all distinction of olfactory 

 nerve and cerebral substance is lost, for there is no trace of 

 any medullary strands or strings of cells that can be regarded 

 as roots to the nerve. After the young crabs reach a length of 

 from 5 to 8 inches the lobes are less conspicuous, breaking up 

 into spindle-shaped masses scattered along the middle third of 

 the nerve. The distal third breaks up into many fine strands, 

 that form a plexus beneath the olfactory buds, from which the 

 buds are supplied. The proximal third shows a more or less 

 clearly marked division into two main strands, corresponding 

 with the olfactory lobes, and each strand goes to its respective 

 cerebral hemisphere. Thus, although I have called this nerve 

 a median nerve, it is in reality a paired one. 



D. Nature of Olfactory Organs. — The primary olfactory 

 thickenings are undoubtedly segmental sense organs serially 

 homologous with the eyes, as first stated in my paper " On the 

 Origin of Vertebrates from Arachnids,^^ p. 337. They cor- 

 respond exactly in position with the lateral eyes of scorpions 

 (PI. 5, figs. 58 and 59), and in their histological structure they 

 show traces of ommatidia and retinal rods, like those in the 

 lateral and median eyes of Limulus. The degeneration of this 

 pair of eyes in Limulus was due probably to their being retained 

 on the under side of a broad shield-like carapace, where they 

 could be of little or no use. Their gradual degeneration and 

 conversion into ganglion-cells, and their subsequent incor- 

 poration with a new set of sense organs with an entirely 



