Fig. 151.—Under Surface of Scorpion (Androctonus).
The operculum is marked out with dots, and on each side of it is seen one of the pectens.
The pecten of the scorpions is an elaborate sense-organ, or rather group of sense-organs, the special organ being developed on each tooth of the comb; its surface, which is frequently flattened, being directed backwards and inwards, when the axis of the pecten is horizontal at right angles to the length of the body. The surface view of this part of the tooth resembles that of the branchial organs or of the flabellum in Limulus, in that it is thickly covered with circular patches, in the centre of which an ill-defined appearance as of a fine pore is seen. In Fig. [152], B, I give a sketch of the surface view of a part of the organ.
Transverse sections of a tooth of the comb of Scorpio Europæus present the appearance given in Fig. [152], A, and show that each of these circular patches is the surface-view of a goblet-shaped chitinous organ, Fig. [152], C, from the centre of which a short, somewhat cylindrical chitinous spike projects. Within this spike, and running through the goblet into the subjacent tissue, is a fine tubule. The series of goblets gives rise to the appearance of the circular plaques on the surface-view, while the spike with its tubule is the cause of the ill-defined appearance of the central pore, just as the terminal pore is much less conspicuous on surface-view in the spike-organs of the flabellum than in the purely poriferous organs, no part of which projects beyond the level of the chitinous surface.
Fig. 152.—A, Section through Tooth of Pecten of Scorpion; B, Surface View of Sense-Organs; C, Goblet of Sense-Organ more highly magnified.
bl. and n., region of blood-spaces and nerves; gl., ganglion-cell layer; ch., modified chitinous layer; s.o., sense-organ.
The fine tubule is soon lost in the thickened but soft modification of the chitinous layer (ch.) which is characteristic of the sense-organ; at all events, I have not succeeded in tracing it through this layer with any more success than in the corresponding case of the tubules belonging to the smaller goblets of the branchial sense-organ of Limulus already described.
At the base of the modified chitinous layer a series of cells is seen, many, if not all, of which belong to the chitinogenous layer. Next to these is the marked layer of ganglion-cells (gl.), similar to those seen in the flabellum of Limulus. The rest of the space in the section of the tooth is filled up with nerves (n.) and blood-spaces (bl.) just as in the section, Fig. [146], of the flabellum of Limulus.