268 THE ORIGIN OF VERTEBRATES 
3. If, as has been supposed, there is a genetic connection between 
(1) and (2) and if, as I suppose, the vertebrates did not arise from 
the annelids directly, but from a protostracan group, then it follows 
that the lateral sense-organs, one of which gave rise to the auditory 
organ, must have been situated on the protostracan appendages. 
4. In Limulus, which is the sole surviving representative of the 
palostracan group, such special sense-organs are found on both the 
prosomatic and mesosomatic appendages, and therefore may be 
expected to give a direct clue to the origin of the vertebrate auditory 
organ. 
5. Both from its position, its size, and its specialization, the 
flabellum, 7.¢. an organ corresponding to the flabellum, must be 
looked upon as more likely to give a direct clue to the origin of the 
auditory organ than the sense-organs of the branchial appendages, or 
the so-called gustatory organs of the mandibles. 
Tue AUDITORY ORGANS OF ARACHNIDS AND INSECTS. 
The difficulty of the investigating these organs consists in the fact 
that so little is known about them in those Arthropoda which live in 
the water; the only instance of any organ apparently of the nature 
of an auditory organ, is the pair of so-called auditory sacs at the base 
of the antenne in various decapods. We are in a slightly better 
position when we turn to the land-living arthropods ; here the pre- 
sence of stridulating organs in so many instances carries with it the 
necessity of an organ for appreciating sound. It has now been shown 
that such stridulating organs are not confined to the Insecta, but are 
present also in the scorpion group, and I myself have added to their 
number by the discovery of a distinct stridulating apparatus in 
various members of the Phrynide. We may then take it for granted 
that arachnids as well as insects hear. Where is the auditory organ ? 
Many observers believe that certain surface-organs found uni- 
versally among the spiders, to which Gaubert has given the name of 
lyriform organs, are auditory in function. His investigations show 
that they are universally present on the limbs and pro-meso-sternite 
of all spiders; that they are present singly, not in groups, on the 
limbs of Thelyphonus, and that a group of them exists on the second 
segment of each limb in the members of the Phrynus tribe. In the 
latter case this organ is the most elaborate of all described by him. 
