364 THE ORIGIN OF VERTEBRATES 



This tube is conical in shape, its-base, which rests on the pigmented 

 layer, being so large and the organs so crowded together that a section 

 of the chitin across the base of the tubes gives the appearance of a 

 honeycomb, the septa of which is all that remains of the chitin. 

 This large tube narrows down to a thin elongated neck as it passes 

 through the chitin, and then, at its termination, bulges out again 

 into an oval swelling (cap.) situated always beneath the homogeneous 

 most external layer of chitin. Within this tube a fine chitinous 

 tubule (eh. t.) is situated similar to that seen in the branchial sense- 

 organs ; it lies apparently free in the tube, not straight, but sinuous, 

 and it passes right through all the chitinous layers to open at the 

 surface as a pore ; in the last part of its course, where it passes 

 through the most external layer (1) of chitin, it lies always at right 

 angles to the surface. 



If the flabellum be stained with methylene blue and acid fuchsin, 



then all the canaliculi in the chitin show up as fine red lines, and 



present the appearance given in Fig. 148, and it is seen that each 



of the terminations of the tubules is surrounded in the homogeneous 



layer of chitin by a thick- set circular patch of canaliculi which pass 



to the very surface of the chitin, while the canaliculi in other parts 



terminate at the commencement of the homogeneous layer and do 



not reach the surface. Further, the contents of the oval swelling, 



and, indeed, of the tube as a whole, are stained blue, the chitinous 



tubule being either unstained or slightly pink in colour. We see, 



then, that the chitinous tubule alone reaches the surface, while the 



large tube, which contains the tubule, terminates in an oval swelling, 



which often presents a folded or wrinkled appearance, as in Fig. 149 



(see also Patten's Fig. 1, Plate I.). This terminal bulging of the 



tube is reminiscent of the bulging in the chitinous tubes of the lyri- 



form organs of the Arachnida, as described by Gaubert, and of the 



poriferous chordotonal organs in insects, as described by Graber (see 



Fig. 150). This terminal swelling is filled with a homogeneous 



refringent mass staining blue with methylene blue, in which I have 



seen no trace of a nucleus ; through this the chitinous tubule makes 



its way without any sign of bulging on its part. Patten, in his 



description of the sense-organs on the mandibles of Limulus, which 



are evidently the same in structure as those on the flabellum, refers 



to this homogeneous mass as a cuagulum. I doubt whether this 



is an adecpaate description ; it appears to me to stain rather more 



