_bl._, blood-spaces; _n._, nerve; _gl._, layer of ganglion-cells; _p._, pigment layer; _ch._, 1, 2, 3, the three layers of chitin; _ch.t._, chitinous tubule in large tube of sense-organ; _cap._, capitellum or swollen extremity of large tube; _can._, very fine porous canals or canaliculi of chitin.] In Fig. 148 I give a magnified representation of a section through three of these flabellar sense-organs. As is seen, the section divides itself into four zones: (1) the chitinous layer (_ch._); (2) the layer of pigment (_p._) and hypodermal cells; (3) the layer of ganglion-cells (_gl._); and (4) the layer of nerve-fibres (_n._) and blood-spaces (_bl._). The chitinous layer is composed of the usual three zones of the Limulus surface--externally (Fig. 148), a thin homogeneous layer, followed by a thick layer of chitin (3), in which the fine vertical tubules or canaliculi are well marked; the external portion (2) of this layer is differentiated from the rest by the presence of well-marked horizontal layers in addition to the canaliculi. In this chitinous layer the special sense-organs are found. They consist of a large tube which passes through all the layers of the chitin except the thin homogeneous most external layer. {364}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. 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.