3 68 THE ORIGIN OF VERTEBRATES 



3 



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 

 palseostracan 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, i.e. an organ corresponding to the flabelluin, 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. 



The Auditoey 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 antenna? 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 Phrynidre. 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. 



