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 lyriform 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 {365}readily than a blood-coagulum, yet in the sense of being structureless it resembles a coagulum. The enormous number of these organs crowded together over the whole flat surface of the flabellum produces a very striking appearance when viewed on the surface. Such a view presents an appearance resembling that of the surface-view of the branchial sense-organs; in both cases the surface is covered with a great number of closely set circular plaques, in the centre of each of which is seen a well-marked pore. The circular plaques in the case of the flabellum are much smaller than those of the branchial sense-organs, and clearly are not protrusible as in the latter organs, the appearance as of a plaque being due to the ring of thickly-set canaliculi round the central tubule, as already described. When stained with methylene blue, the surface view of the flabellum under a low power presents an appearance of innumerable circular blue masses, from each of which springs a fine bent hair, terminating in a pore at the surface. The blue masses are the homogeneous substance (_cap._) of the bulgings seen through the transparent external layer of chitin, and the hairs are the terminal part of the chitinous tubules. Patten has represented their appearance in the mandibles in his Fig. 2, Plate I.