MORPHOLOGY OF BRAIN AND SENSE ORGANS OP LIMULUS. 21 



getting such perfect isolation as with the gustatory cells, and 

 there still remain some points of importance unanswered. But 

 the cardinal point at issue, whether the buds are glandular or 

 sensory, was settled beyond doubt, for I was able to demon- 

 strate that there is rarely a lumen in the fully formed buds, 

 and that when it does occur it does not communicate with the 

 exterior. Moreover I proved that the chitinous tubule cannot 

 be a duct to the gland, since it is in reality a direct pro- 

 longation of the central ganglion-cell, and may be 

 compared with the tubule in the distal end of the 

 gustatory cells. Thus every reason for regarding these 

 sense buds as glands disappears. 



The lumen of the buds varies greatly in its appearance. It 

 is most commonly present in the newly formed buds, where it 

 is spherical and sharply circumscribed (fig. 23). In the adults 

 it seldom has this appearance, and may be entirely absent, or 

 it may be reduced to a small irregular space between the cells. 

 Although I have looked carefully for them I have never seen 

 any of the clear globules of the gland-like cells in the lumen 

 of the gustatory buds. The lumen appears to be something 

 like a much-retarded invagination cavity, although, as nearly 

 as I can make out, the organs arise by a solid ingrowth from 

 the ectoderm. 



The tubule can be followed in sections from near the centre 

 of the bud, through the cuticular canals, to a point very near 

 the outer surface ; here it becomes very faint or disappears. It 

 may be either straight, very much coiled, broken at intervals as 

 though it were very brittle, or may have one or more spindle- 

 shaped swellings. The tubule is undoubtedly composed of 

 chitin, for, as with tlie gustatory tubules, they can still be 

 seen in the cast-off shells of immature specimens and in the 

 fresh shells cleaned with potash. 



The cuticular canals of the olfactory buds are easily dis- 

 tinguished from the gustatory ones by their shape. Each 

 canal in the adult has a nearly uniform diameter, except that 

 near the outer surface it suddenly narrows and communicates 

 with the exterior by a transverse slit (figs. 9 and 12, C and D). 



