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 (ch, ¢.) 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 coagulum. I doubt whether this 
is an adequate description; it appears to me to stain rather more 
