The Origin of Vertebrates

by Walter Holbrook Gaskell

Fig. 146.—Section through Flabellum. ch., chitinous layers; s.o., sense-organs; sp., spike-organ; p., pigment layer; gl., ganglion cell layer; bl. and n., blood-spaces and nerves.

Fig. 147.—Section parallel to the Surface of Flabellum, showing the Porous Terminations of the Sense-Organs and the Arrangement of the Canaliculi round them.

At present I am inclined to look upon this mass of cells as constituting a large ganglion, which extends over the whole length and breadth of the upper surface of the flabellum. At the same time, my preparations are not sufficiently clear to enable me to trace out the connections of these cells, especially their connections with the special sense-organs.

Fig. 148.—Section through the three Sense-Organs of Flabellum.

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. 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.

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 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.

The large tubes in the chitin alter in shape according to their position. Those in the middle of the sensory surface of the flabellum, in their course through the chitinous layers, are hardly bent at all; as they approach the two lateral edges of this surface, their long thin neck becomes bent more and more, the bending always being directed towards the middle of the surface (see Fig. [146]); in this way the chitinous tubules increase more or less regularly in length from the centre of the organ to the periphery. The large basal part of the conical tube contains, besides the chitinous tubule, a number of nuclei which are confined to this part of the tube; some of these nuclei look like those belonging to nerve-fibres, others are apparently the nuclei of the chitinogenous membrane lining the tube. I have never seen any sign of nerve-cells in the tube itself.