164 THE OLFACTORY ORGANS AND THE OLFACTORY LOBES. 



II. The Olfactory Lobes of Arachnids. 



Development. — The olfactory lobes (organe stratifie, St. Remy) are prob- 

 ably present in all arthropods. They always form a conspicuous part of the fore- 

 brain in arachnids, but their functions and their relations to other parts of the 

 procephalon are unknown, except in Limulus where they are associated with the 

 olfactory nerves; their function is thus definitely indicated. It is singular that 

 Limulus is also the only form in which the lobes come into close morphological 

 relation with the nerve roots to the median ocelli. 



In the scorpion and in spiders, the olfactory lobes arise from the walls of a 

 deep transverse groove extending across the anterior end of the medullary plate. 

 The groove probably represents the whole of the first neuromere, hence they repre- 

 sent the very anterior margin of the primitive nerve axis. (Figs. 15, 16, 20, 46.) 



In the later stages, the groove closes and its walls form a conspicuous crescentic 

 band of small, deeply stainable cells, on the anterior haemal aspect of the fore- 

 brain, Figs. 41, 42 ol.l. 



The Olfactory Lobes of Limulus. 



Development. — In Limulus, the olfactory lobes appear as two separate in- 

 foldings. (Figs. 141, 142, ol.l.) Later the lobes unite and migrate backward 

 over the haemal surface of the brain, gradually changing from a thick, bi-lobed 

 transverse bar extending across the very anterior end of the brain, to an elongated 

 U-shaped disc lying on its haemal surface. (Fig. 36.) In the adult, the posterior 

 margin of the bow extends backward, well below the middle of the cheliceral seg- 

 ment, farther back than its position in the half grown specimens shown in 

 Figs 47, 5, 48, 51. 



The lobe is formed from the posterior wall of the original infolding, the mem- 

 branous anterior wall disappearing during the later stages. 



The entire margin of the lobes consists of very small, closely packed cells 

 resembling the granule cells of the cerebral cortex. As they freely absorb all 

 kinds of nuclear stains, the outlines of the lobes can usually be seen with great 

 distinctness. 



In young Limuli the anterior arms of the bow-shaped lobes are drawn together, 

 forming two slender horns which up to the late larval stages, are continuous with 

 the lips of the anterior neuropore. (Fig. 36, A and B.) At about the time the 

 neuropore closes (after the trilobite stage) there is a vigorous forward outgrowth 

 at this point, apparently originating in the hemispheres. This forward outgrowth 

 carries with it the pointed ends of the olfactory lobes and the peculiar tissue of the 

 hemispheres, giving rise to the median olfactory nerve and its ganglion. Thus 

 the stalk of the median olfactory is continuous with both hemispheres and with 

 both horns of the olfactory lobes. 



Structure. — As the crabs grow older, the cells on the median portions of the 

 lobes become very large, and divide into several distinct clusters. All the marginal 



