THE EVIDENCE OF THE AUDITORY APPARATUS 361 
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 (¢f. 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 
