120 GENERAL AND SPECIAL CUTANEOUS SENSE ORGANS. 



ance of mucous in the olfactory and gustatory organs, and for its absence in the 

 tactile or auditory ones. 



The mandibular slime buds are sufficiently numerous to suggest that they 

 are in the nature of salivary glands. This, however, does not seem probable, since 

 there is no way to get the secretions into the mouth with the food; and the mem- 

 branes immediately within, or surrounding the mouth are entirely devoid of these 

 organs. Moreover, it is certain that the precisely similar slime buds in the ol- 

 factory fields, and in the integument of the back or branchial chamber, cannot be 

 regarded as salivary organs. 



7. The slime buds of Limulus and other arachnids are found in segmentally 

 arranged fields, or groups, that are supplied by special nerves, the most conspicuous 

 groups being those in the olfactory organ, in the mandibles of the second to the 

 fifth thoracic appendages, and in the rudimentary vagus appendages (scorpion). 

 These organs appear at an early embryonic period as thickenings of the ectoderm 

 and in close association with the cranial ganglia. 



The Auditory Organ. In my first contribution, 1889, I maintained that 

 the large segmental sense organ, which in Limulus embryos lies opposite the fourth 

 pair of legs, was the probable forerunner of the vertebrate ear. I see no reason 

 to change my opinion on this point. Although the evidence in favor of this con- 

 clusion is not voluminous, it is sufficiently precise as far as it goes. In Limulus, 

 the organ in question is a large discoidal placode, of a sensory nature (Figs. 131, 

 1 40 to 153), strikingly like the auditory placode of vertebrates in its general outward 

 appearance, in its minute structure, and in the fact that it is located, as nearly as 

 one may determine, on the same segment of the head, using as a guide either 

 the history of the oral arches (Figs. 29-34), or the number of the corresponding 

 brain neuromere. (Fig. 57.) 



It is assumed that in the primitive vertebrates this particular placode, which 

 lies at the head of the posterior division of the thorax, formed a simple, sac-like 

 infolding, similar to the auditory sac in decapods, and that from this sac developed 

 the inner ear of vertebrates. 



The placode belongs to the same series as the visual and olfactory organs. 

 (Figs. 140-148, 5.o 4 .) It increases in size up to the time of hatching. During the 

 early trilobite stage, the cells become slightly pigmented, take on a sensory ap- 

 pearance, and a lens-like thickening of the overlying chiten is formed over it. 

 (Fig. 131, B.) The organ disappears completely at the close of the trilobite stage. 



That is as far as the evidence goes. There is no evidence that the placode in 

 Limulus is auditory; or that it is serially homologous with the antennal auditory 

 organs of decapods, although that is not improbable. 



Gaskell regards the flabellum of Limulus, or the pectines of the scorpion, as 

 the precursor of the vertebrate auditory organ; but they lie much too far back in 

 the head to be compared with the ear of vertebrates. His description of the 

 minute structure of the flabellum is very inaccurate, and his intimation that it is 



