Passing now from the branchial to the prosomatic region, the first thing that struck me was the presence of that most conspicuous projection at the base of the last locomotor appendage, which is usually called the flabellum, and has been described by Lankester as an exopodite of this appendage. It is jointed on to the most basal portion of the limb (_cf._ Fig. 155), and projects dorsally from the limb into the open slit between the prosomatic and mesosomatic carapace, as is seen in Fig. 145 (_fl._). Of its two surfaces, the undermost is very convex and the uppermost nearly flat from side to side, the whole organ being bent, so that when the animal is lying half buried in sand, entirely covered over by the prosomatic and mesosomatic carapaces except along this slit between the two, the upper flat or slightly convex surface of the flabellum is exposed to any movement of water through this slit, and owing to its possessing a joint, the direction of the whole organ can be altered to a limited extent. The whole of this flat upper surface is one large sense-organ of a striking character, thus forming a great contrast to the convex under surface, which is remarkably free from tactile spines or special sense-organs. The nerve going to the flabellum is very large, almost as large as the nerve to the rest of the appendage, and the very large majority {362}of the nerve-fibres turn towards the flat, uppermost side, where the sense-organ is situated. Between the nerve-fibres (_n._) and the chitinous surface containing the special sense-tubes masses of cells (_gl._) are seen, as in Fig. 146, apparently nerve-cells, which form a broad border between the nerve-fibres and the pigmented chitinogenous layer (_p._). On the opposite side, nothing of the sort intervenes between the pigmented layer and the blood-spaces and nerve-fibres which constitute the central mass of the flabellum. [Illustration: 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.] [Illustration: 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 {363}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. [Illustration: FIG. 148.--SECTION THROUGH THE THREE SENSE-ORGANS OF FLABELLUM.