S6o 



NA TURE 



[Oct(m;er 8, 1896 



Ijf consideix-d with the ciinsideralion 01 Ihe old moulliitube. 

 The comparison of the metastoina of Euryplenis with the lower 

 lip of Ammoccetes demonstrates the close resemblance between 

 the oral chambers of Kurypterus and Ammoccetes. In order to 

 obfain the condition of affairs in Ammoccetes froiti that in 

 Euryptenis, it is only necessary that the metastoma should 

 increase in size, and that the last oral appendage, the large 

 oar-appendage, should follow the example of the other oral 

 appendages, and be withdrawn into the oral cavity, and so form 

 the velar appendage. 



Thus we see that, jtist as the mesosomatic apjiendages of 

 LimuUis can be traced into the branchial and thyroid appendages 

 of Ammoccetes through the intermediate stage of forms similar 

 to Kurypterus, .so also the prosomatic appemlages and chilaria 

 of Limulus can be traced into the velar and tentacular append- 

 ages and lower lip of Ammoccetes through the intermediate 

 stage of forms similar to Kurypterus. 



3. Lastly comes Ihe ontogenetic test. The concordant inter- 

 pretation of the origin of the motor part of the Vth, of the 

 Vllth, IXth, and Xth nerves given by the anatomical and 

 phylogenetic tests must explain and be illustrated \fj the facts 

 of the development of Ammoccetes. 



We see : — 



(1) The oral chamber of Ammoccetes is known in its early 

 stage by the name of the stomatodieum, and we find, as might 

 be anticipated, that it is completely separated at first from the 

 branchial chamber by the septum of the stoniatodoeum. 



(2) This septum is the embryological representative of the 

 basal part of the operculum, and demonstrates that originally 

 the operculum separated the oral and branchial chambers. 



(3) Subsequently these two cliambers are put into communi- 

 cation l)y the breaking through of this septum, illustrating the 

 communication between the two cliambers by the separation of 

 the median basal parts of the operculum. 



(41 The velar appendages, the tentacular appendages, the 

 lower lip, all form as out-buddings, just as the homologous 

 locomotor appendages are formed in arthropods. 



(5) The branchial liars are not formed by a series of inpouch- 

 ings in a tube of uniform thickness, but, as Shipley (he. cit.) 

 has pointed out, by a series of ingrowths at regular intervals; 

 in other words, the embryological history represents a series of 

 buddings — i.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 lube 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 conceivable, but cannot at present be 

 explained with the same certainty as most of the embryological 

 facts of vertebrate development. I can only say that Striilael 

 \Zool. Anzeiger, vol. xv. , 1S92) has pointed out, and I can con- 

 firm him, that after the young Thelyphonus has left the egg, 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 communicate with the exterior. 



(7) The branchial cartilages in the Ammoccetes are formed 

 distinctly before the auditory capsules and trabecula;, illustrative 

 of the fact that they alone are formed in Limulus. 



Compan.wn of the Auditory Apparatus of Aininocates with the 

 Flabellum of Limulus. Meaning of the Vlllth Neii'e. 

 The correctness of a theory is tested in two ways: (i) 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 pro.somatic and mesosomatic regions of the 

 body in Limulus and the scorpions are comparable with the 

 corresponding regions of Ammoccetes as far as their locomotor 

 and branchial appendages are concerned, and that, therefore, a 

 satisfactory explanation is given of the peculiarities of the Vth, 

 Vllth, IXth, and Xth nerves. In all vertebrates, however, there 

 is invariably found a special nerve, the Vlllth 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 Vlllth nerves 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 jirosomatic and meso- 

 somatic appendages, at the very base of the last prosomatic 

 appendage. At present we know nothing aliout the nature or 

 locality of the hearing apparatus of Limulus. It is, therefore, 



NO. 1406, VOL. 54] 



all the Tnoie interesting to find that in tlie very posilicm de- 

 manded by the theory, at the base of the last prosomatic ap- 

 pendage, is found a large hemispherical organ, to which a mov- 

 able spatula-like process is attached, known by the name of the 

 fialn-llum. This organ is confined to the base of this limb ; it is 

 undoubtedly a special .sense-organ, being composed mainly ol 

 nerves, in connection with an elaborate arrangement of cellsand 

 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 (Ann. d. Sci. Nat., Zool., 7th ser., 

 tome 13, 1892), 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 side- 

 wards, 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 Animocoetes is found in the fact that in each case the nerve is 

 accoinpanied into the capsule by a diverticulum of the liver and 

 generative organs. (See tintted sub.stance in Figs. 4 and 6). In 

 Limulus the liver and generative organs, which surround the cen- 

 tral nervous system from one end of the body to the other, do 

 not penetrate into any of the appendages, with Ihe single excep- 

 tion of \he flahellum. 



In Ammocietes the peculiar glandular and pigmented tissue 

 which surrounds the brain and spinal cord, and has already been 

 recognised as the remains of the liver and generative organs, does 

 not penetrate into the velar or other append.ages, but is found 

 only in the auditory capsule, where it enters with and partly 

 surrounds the auditory nerve. 



The coincidence is so startling and unexpected as to bring 

 conviction to my mind that in the fiahellum of Limulus we are 

 observing the origin of the vertebrate auditory apparatus ; and 

 it is, lo say the least of it, suggestive that in (laleodes the last 

 locomotor appendage should carry the extraorilinary racquet- 

 shaped organs which Gaubert has shown to be sense-organs of a 

 special character, and that in the scorpion a large special sen.se- 

 organ of a corresponding character, viz. the pecten, should be 

 found which, from its innervation, as given by Patten (Quart, 

 /ouin. of Micr. Sci., vol. xxxi., 1890), appears to belong to the 

 segment immediately anterior to the operculum, rather than to 

 that immediately posterior to it. 



Comparison of the Olfactory Organ of Ammocir/es with the 

 Camerosiome of Thelyphonus. Meaning of the /si Nenv. 

 .Also comparison of the Hypophysis with the Mouth-tube of 

 7'he/rphonus. 



In precisely the same way as the theory has led to the dis- 

 covery of a special sense-organ in Limulus and its allies which 

 may well lie auditory, so also it must lead to the discovery ot 

 the olfactory apjiaratus of the same group, for here also, just as 

 in the case of the auditory apparatus, we are at present entirely 

 in the dark. 



The olfactory organ in such an animal as Thelyphonus ought 

 to be innervated from the supra-cesophageal ganglia, and ought 

 to be situated in the middle line in front of the mouth. The 

 mouth is at the anterior end in these animals, the lower lip or 

 hypostoma (see Fig. 9) being formed by the median projecting 

 flanges of the basal joints of the two pedipalpi ; above, in the 

 middle line, is a peculiar median appendage called the camer- 

 ostome. Still more dorsal we find in the median line the rostrum, 

 with the median eyes near its extremity, and laterally on each side 

 of the camero.stome, and dorsal to it, are situated the p0werfi.1l 

 chelicerx, which are considered by some authorities to represent 

 antennae. Of these parts the camerostome is certainly innervated 

 from the supra-cesophageal ganglia, and upon cutting sagittal 

 transverse sections in a very young Thelyphonus we find that the 

 surface is remarkably covered with very fine sense-hairs, 

 arranged with great regularity and connected with a conspicuous 

 mass of large cells. Upon making transverse sections through 

 this region we see that the camerostome projects into the orifice 

 of the mouth, and that its sense-epithelium lorms, together with 

 a similar epithelium on the lower lip, a closed cavity surrounded 

 by a thick edge of fine hairs. Mere, then, in the camerostome 

 of Thelyphonus is a special sense-organ which, froin its position 

 and its innervation, may well be olfactory in function, or at all 

 events subserve the function of taste. 



