380 THE ORIGIN OF VERTEBRATES 
Here there was a test of the truth of my theory ranking second 
only to the test of the median eyes; the strongest possible evidence 
of the truth of any theory is given when by its aid new and unex- 
pected facts are brought to light. The theory said that in the group 
of animals from which the vertebrates arose, a special sense-organ 
of the nature of an auditory organ must have existed on the hase of 
one of the appendages situated at the junction of the prosoma and 
mesosoma, and that into this basal part of the appendage a portion 
of the cephalic mass of generative and hepatic material must have 
made its way in close contiguity to the nerve of the special 
organ, 
The only living example which nearly approaches the ancient 
extinct forms from which, according to the theory, the vertebrates 
arose, is Limulus, and, as has already been shown, in this animal, in 
the very position postulated by the theory, a large special sense- 
organ—the flabellum—exists, which, as already stated, may well 
have given rise to a sense-organ concerned with equilibration and 
audition. If, further, it be found that a diverticulum of the gene- 
rative and hepatic material does accompany the nerve of the 
flabellum in the basal part of the appendage, then the evidence 
becomes very strong that the auditory organ of Ammoceetes, i.e. of 
the ancient Cephalaspids, was derived from an organ homologous 
with the flabellum; that, therefore, the material round the brain of 
Ammoccetes was originally generative and hepatic material; that, in 
fact, the whole theory is true, for all the parts of it hang together so 
closely that, if one portion is accepted, all the rest must follow. As 
pointed out in my address at Liverpool, and at the meeting of the 
Philosophical Society at Cambridge, it is a most striking fact that a 
mass of the generative and hepatic tissue does accompany the flabellar 
nerve into the basal part of this appendage. Into no other appendage 
of Limulus is there the slightest sign of any intrusion of the gene- 
rative and hepatic masses ; nowhere, except in the auditory capsule, 
is there any sign of the peculiar large-celled tissue which surrounds 
the brain and upper part of the spinal cord of Ammoccetes. The 
actual position of the flabellum on the basal part of the ectognath is 
shown in Fig, 155, A, and in Fig. 155, B, I have removed the chitin, 
to show the generative and hepatic tissue (gen.) lying beneath. 
The reason why, to all appearance, the generative and hepatic 
mass penetrates into the basal part of this appendage only is apparent 
