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 base 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 Ammoccetes, 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 



