THE EVIDENCE OF THE AUDITORY APPARATUS 359 



last locomotor appendage, known as the nabellum, 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 Palseostraca 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.h), 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. 



