MORPFTOLOGY OF BRAIN AND SENSE ORGANS OF LIMULUS. 19 



had eaten into it^ or it may contain a network of membranous 

 canals, evidently the tubes of some Annelid ; they are usually 

 most abundant in the median portion of the olfactory organ, 

 and either lie on the surface or are buried in the cuticula. 



Scattered over the olfactory organ are many blunt back- 

 wardly curved spines. The olfactory cuticula can be easily 

 peeled off in successive layers, but it adheres strongly around 

 the pores leading to these spines. When it does come away 

 large tufts of pigment-cells are seen projecting from the spine 

 pores, and the outer surface of the inner layer of cuticle pro- 

 jects in a crater-like collar around the pore. There are similar 

 spines on the cuticle surrounding the olfactory organ, but they 

 do not act like the ones just described. For this reason T sup- 

 posed at first that the olfactory spines were perhaps the true 

 sense organs supplied by the olfactory nerves, but I can find 

 but little evidence in support of this view. The large pores 

 leading up into the spines over the olfactory organ are lined 

 with thin cells and crowded with pigmented tissue, and in some 

 cases contain a transparent, fragmented coagulum. A chi- 

 tinous tubule, similar to that of the gustatory cells, usually 

 runs the whole length of the canal, and becomes continuous 

 with a minute canal extending from base to summit of the 

 spine. The latter is pinnately striated in section, as though 

 its central canal were connected with the exterior by innu- 

 merable radiating canals. Under favorable conditions a rather 

 large nerve may be seen to enter the base of the spine canal. 

 The spines are suspended in sockets, and are moveable. They 

 are undoubtedly of a sensory nature, but they seem to play 

 only a very subordinate part in the olfactory region, and, con- 

 trary to what 1 at first surmised, cannot be compared with the 

 large gustatory spines on the mandible. 



A section through the olfactory organ (fig. 21) shows the 

 larger branches of the nerve-plexus arising, in the main, from 

 the median olfactory nerve ; also the densely pigmented layer 

 of ectoderm confined to the olfactory region, the clear olfac- 

 tory buds {ol. b.), the small clusters of dark cells looking like 

 ganglia, and numerous branching blood-vessels. One layer of 



