962 - REPORT—1896. 
regular intervals ; in other words, the embryological history represents a series of 
buddings—?.e. appendages within the branchial chamber similar to the buddings 
within the oral chamber—and does not indicate the formation of gill-pouches by 
the thinning of an original thick tube at definite intervals. 
6. The communication of the branchial chamber with the exterior by the 
formation of the gill-slits represents a stage in the ancestral history which is con- 
ceivable, but cannot at present be explained with the same certainty as most of the 
embryological facts of vertebrate development. I can only say that Striibel? has 
pointed out, and I can confirm him, that after the young Thelyphonus has left the 
ege, and is on its mother’s back, before the moult which gives it the same form as 
the adult, the gills and gill-pouches are fully formed, but do not as yet communi- 
cate with the exterior. 
7. The branchial cartilages in the Ammoccetes are formed distinctly before the 
auditory capsules and trabeculee, illustrative of the fact that they alone are formed 
in Limulus. 
Comparison of the Auditory Apparatus of Ammocetes with the Flabellum 
of Limulus. Meaning of the VILIth Nerve. 
The correctness of a theory is tested in two ways:—(1) It must explain all 
known facts; and (2) it ought to bring to light what is as yet unknown, and the 
more it leads to the discovery of new facts, the more certain is it that the theory 
is true. So far, we see that the prosomatic and mesosomatic regions of the body in 
Limulus and the scorpions are comparable with the corresponding regions of 
Ammocecetes as far as their locomotor and branchial appendages are concerned, and 
that, therefore, a satisfactory explanation is given of the peculiarities of the Vth, 
VIilth, 1Xth, and Xth nerves. In all vertebrates, however, there is invariably 
found a special nerve, the VIIIth nerve, entirely confined to the innervation of the 
special sense-organs of the auditory apparatus. It follows, therefore, that if my 
theory is true the VIIIth nerve must be found in such forms as Limulus and its 
allies, and that, therefore, a special sense-organ, probably auditory in nature, must 
exist between the prosomatic and mesosomatic appendages, at the very base of the 
last prosomatic appendage. At present we know nothing about the nature or 
locality of the hearing apparatus of Limulus. It is, therefore, all the more in- 
teresting to find that in the very position demanded by the theory, at the base of 
the last prosomatic appendage, is found a large hemispherical organ, to which a 
movable spatula-like process is attached, known by the name of the flabellum. 
This organ is confined to the base of this limb; it is undoubtedly a special sense- 
organ, being composed mainly of nerves, in connection with an elaborate arrange- 
ment of cells and innumerable fine hairs, which are thickly imbedded in the chitin 
of the upper surface of the spatula. The arrangement of these cells and hairs is 
somewhat similar to that of various sense-organs described by Gaubert,? and 
supposed to be auditory. When the animal is at rest this sensory surface projects 
upwards and backwards into the crack between the prosomatic and mesosomatic 
carapaces, so that while the eyes only permit a look-out forwards and sidewards, 
and the whole animal is lying half buried in the sand, any vibrations in the water 
around can still pass through this open crevice, and so reach the sensory surface of 
this organ. 
Finally, the most striking and complete evidence that this sense-organ of 
Limulus is homologous with the auditory capsule of Ammoccetes is found in the 
fact that m each case the nerve is accompanied into the capsule by a diverticulum 
of the liver and generative organs. (See dotted substance in figs, 4 and 6.) In 
Limulus the liver and generative organs, which surround the central nervous system 
from one end of the body to the other, do not penetrate into any of the appendages, 
with the single exception of the flabellum. 
In Ammoceetes the peculiar glandular and pigmented tissue which surrounds 
' Striibel, Zool. Anzeiger, vol. xv. 1892. 
* Gaubert, Ann. d. Sci. Nat., Zool., Tth ser., tome 13, 1892. 
