THE EVIDENCE OF THE AUDITORY APPARATUS 359 
last locomotor appendage, known as the flabellum, was an elaborate 
sense-organ. I now propose to show that all these special sense- 
organs are constructed on a somewhat similar plan ; that the structure 
of the branchial sense-organs suggests that they are organs for the 
estimation of water pressures ; that among air-breathing arthropods 
sense-organs, built up on a somewhat similar plan, are universally 
found, and are considered to be of the nature of auditory and equi- 
libration organs; and, what is especially of importance, in view of 
the fact that the most prominent members of the Paleostraca were 
the sea-scorpions, that the remarkable sense-organs of the scorpions 
known as the pectens belong apparently to the same group. 
THE PoRIFEROUS SENSE-ORGANS OF THE APPENDAGES IN LIMULUS. 
On all the branchial appendages in Limulus, special sense-organs 
are found of a most conspicuous character. They form in the living 
animal bluish convex circular patches, the situation of which on the 
appendages is shown in Fig. 58. These organs are not found on the 
non-branchial operculum. Gegenbaur, who was the first to describe 
them, has pointed out how the surface of the organ is closely set 
with chitinous goblets shaped as seen in Fig. 144, A, which do not 
necessarily project free on the surface, but are extruded on the 
slightest pressure. Each goblet fits into a socket in the chitinous 
covering, and is apparently easily protruded by variations of pressure 
from within. The whole surface of the organ on the appendage is 
slightly bulged in the living condition, and the chitin is markedly 
softer here than in the surrounding part of the limb. Each of these 
organs is surrounded by a thick protection of strongly branching 
spines. On the surface of the organ itself no spines are found, only 
these goblets, so that the surface-view presents an appearance as in 
Fig. 144, B. Each goblet possesses a central pore, which is the 
termination of a very fine, very tortuous, very brittle chitinous 
tubule (ch.t.), which passes from the goblet through the layers of. the 
chitin into the subjacent tissue. The goblets vary considerably in 
size, a few very large ones being scattered here and there. The fine 
chitinous tubule is especially conspicuous in connection with these 
largest goblets. In the smaller ones there is the same appearance of 
a pore and a commencing tube, but I have not been able to trace the 
tube through the chitinous layers, as in the case of the larger goblets. 
