MORPHOLOGY OF BEAIN AND SENSE OEGAKS OF LIMULUS. 13 



dibular Spines. — It is not difficult to find the three sets of 

 gustatory organs, the existence of which is demonstrated above. 

 Examination under a low magnifying power shows that the man- 

 dibular spines are thickly covered on their sides nearest the 

 mouth with minute pores arranged in from eight to ten inter- 

 rupted vertical lines (fig. 2). Each line consists of several 

 subordinate groups composed of from two to twelve or more 

 pores. Longitudinal sections of one of the spines (fig. 1) show 

 that the cuticula is perforated by parallel canals, in each of 

 which is a delicate chitinous tubule {ch. t.); the latter 

 terminates at the outer opening of the canal flush with the 

 surface ; at the opposite end it bends nearly at right angles 

 towards the base of the spine, where it soon expands into a 

 clear, spindle-shaped body [sp.) ; beyond the spindle it is 

 continued as a very long slender process that constitutes the 

 body of the gustatory cell, the nucleated end of which unites 

 with other cells to form great ganglion-like masses [gsc). 



The spindle, when more highly magnified (fig. 5), seems 

 to be merely an inflation of the cell-wall, and contains, besides 

 a watery fluid, a number of fibrillse, arranged in a single layer 

 along its inner wall, and continuous with fibrillse in the stalk of 

 the cell. The proximal half of the spindle is usually stained a 

 little darker than the rest, and in it each fibril expands into a 

 fusiform thickening that stains deeply in acetic acid carmine. 

 The fibrillse converge toward the distal half of the spindle to 

 form an axial bundle that can be followed nearly to the free 

 outer end of the chitinous tubule. From two to eight slender 

 bipolar cells surround each spindle, and send their fibrous pro- 

 cesses outward along the outer surface of the chitinous tubule 

 (figs. 5, 6,g.c.). 



It is difficult to tell just where the tubule begins. A short 

 distance beyond the spindle (figs. 5 and 6) it appears to be 

 continuous with the cell-wall, which there becomes rather 

 distinct owing to its separation from the cell contents. It is 

 slightly indented in some places, and is apparently enclosed 

 by a second membrane, probably the outward prolongation 

 of the nerve-like cells surrounding the spindle, for when 



