THE EVIDENCE OF THE AUDITORY APPARATUS 36 1 



brought below the level of the surface, and he imagines that the pro- 

 trusion is effected by hydraulic means, by the aid of the vascular 

 system. In the branchial sense-organs of Limulus there are no 

 retractor muscles, and it seems to me that both retraction and pro- 

 trusion must be brought about by alterations of pressure in the 

 vascular fluids. Certainly the cavity of the organ is very vascular. 

 If this be so, it seems likely enough that such an organ should be a 

 very delicate organ for estimating changes in the pressure of the 

 external medium, for the position of the goblets would depend on 

 the relation between the pressure of the fluid inside the organ and 

 that on the surface of the appendage. Whether the chitinous tubule 

 contains a nerve-terminal or not I am unable to decide from my 

 specimens, but, judging from Patten's description of the similar 

 chitinous tubules belonging to the mandibular organs, it is most 

 highly probable that these tubules also contain a fine terminal 

 nerve-fibre. 



These organs, then, represent segmental branchial sense-organs, 

 of which it can be said their structure suggests that they may be 

 pressure-organs ; but the experimental evidence is at present wanting. 



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 (_/?.). 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 appe adage, and the very large majority 



