COMPAEATIVE MOEPHOLOGr OF THE GALEODID^.. 351 



legs. There may also he some difference in the length of the shaft in males and 

 females. 



In sagittal sections the sensory processes of the pectincs of Scorpio are seen to belong to the ventral, 

 if not to the anterior, face of the limb, which is folded backward. They are not on the posterior face, 

 where they ought to be if they were the derivatives of gills like those of Limulus. 



The shaft suddenly expands into a fan, which is apparently capahle of hcnding on the 

 former. The more median fans are symmetrical with the shaft ; those lying externally 

 are asymmetrical, the inner or median edge heing almost on a line with the shaft. 



The distal edge of the fan has a fold rvmning along its wliole length. The investiga- 

 tion of the exact natui'C of this fold has heen a task of some difficnltv. The foUowinsr 

 description differs from that given hy Gauhert (32), whose account seems to me rather 

 too simple and diagrammatic. It is true that his drawings refer to the racquet-organs of 

 Odleodes barbarus, whereas mine refer to those of Galeodes arabs and Rhax fsp.). 



Compared w'ith the depth of the whole fan (excluding the shaft) the fold is very 

 shallow, -2^0- to -0-5-, and not ,V, as in Gauhert's drawing *. The fold appears to he very 

 similar in Mhax and Galeodes ; the form in section can he gathered from PI. XXXII. 

 fig. 2. We have a sharp, stiff ridge (in cross-section a digitiform process), bordering the 

 furrow anteriorly, while the posterior hoimdary is holster-shaped (in section a knoh-like 

 process), which either stands straight up or hends over towards the stiff digitiform 

 process (PI. XXXII. fig. 3). 



The nerve-endings in the specimens examined by me are not whei-e Gauhert figiu^es 

 them, ^. e. in the base of the fvuTow, but on the bolster-shaped ridge, in such a position 

 that when it stands up so that the fiu-row is open the nerve-endings are along the most 

 distal edge of the ridge ; when, however, the bolster rolls inward against the sharp, 

 stiff ridge, the nerve-endings would be protected by this latter ridge {of. the figures). 



The chitin of the whole racquet-organ is very thick and tough, and my attempts to 

 make thin sections almost completely failed. The lateral edges of the fans are specially 

 thickened (PI. XXXII. fig. 4, ch). On the flat surfaces the ordinary staining layers of the 

 cuticle can be made out, and an outermost refractive layer which has a very wavy 

 external surface. The waviness is very pronounced near the furrow-edge of the fan, and 

 may be due to the movement of the two ridges bordering the furrow in opening out and 

 closing one against another. An irregular row of markings occurs on each face of the 

 fan ; these are apparently apertures for the passage of fine sensory hairs, which are, as a 

 rule, destroyed. I have, how^evcr, found one or two m situ (cf. fig. 4, sh). 



Internally the racquets show the following arrangement of tissue : — The powerful 

 nerve, whose fibres are closely mingled with fine tracheal tubes, spreads out like a fan 

 on leaving the shaft. The nerve-fibrils appear to run distinct, bathed in blood-fluid, for, 

 between the fibrils, blood-ceUs are freely scattered. The fibrils themselves have long 

 fusiform nuclei in their course ; whether more than one to a fibril, and if so at what 

 distance apart, I have been imable to make out (PI. XXXII. fig. 1, nf.). 



After traversing about halfway through the depth of tlie fan, the nerve-fibres no longer 



* Plate 3. fig. 5. It is necessary to refer to Gauhert's details, as his account is, so far as I know, the first 

 attempt to describe the minute anatomy of these organs. 



