OCTOHEK 8, 1896J 



NA TURE 



559 



I of the veiil 

 appendagi 



NO. 1406, VOL. 54] 



face of the branchial region, 

 arketl out by its dotted appearance. 



wall.s a jjlaiidiilar slnicture which resembles the tli)iuia rrlantl ot 

 .\mmoccetes in a remarkable degree. A section is repiesented 

 in Fig. 7, and we see that under the operculum in the middle 

 line is situated a ttibe, the walls of which in one part on each 

 side are thickened by the formation of a gland with long cells ot 

 ihe same kind as those of the thyroid ; the nucleu? is spherical, 

 and situated at the farther enil of the cell, and the cells are 

 arranged in wedges, .so that the e.xtreniities of each group ot 

 cells come to a point on the surface of the inner lining of the 

 lube. This point is marked by a small round opening in the in- 

 ternal chilinous lining of the tube. These cells form a column 

 along the whole length of the tube, ju^t as in the thyroid gland, 

 so that the chitinous lining along that column is perforated by 

 numbers of small round holes. This glandular stiuclure is not 

 confined to the male scorpion, but is found also in the female, 

 I hough not so well developed. 



So characteristic is ihe structure, so different from anything 

 else, that I have no hesitation in saying that the thyroid ot 

 .Ammoccetes is the same structurally as the thyroid of the 

 scorpion, and that, therefore, in all probability the median 

 projection of the operculum in the old forms of scorpions, such 

 as Eurypterus, Pterygotus, Slimonia, &c., covered a glandular 

 tube of the same nature as the thyroid of Amniocretes. 



We see, then, that the structures innervated by the Vllth, 

 IXth, and Xlh nerves are absolutely concordant with the view 

 that the primitive vertebrate respiratory chamber was formed 

 from the mesosoniatic appendages of such a form as Limulus by 

 a slight modification of the method by which the respiratory 

 apparatus of Thelyphonus and other Arachnids has been formed, 

 according to Macleod. The anterior limit of this chamber was 

 formed by the operculum, the basal part of which formed a 

 septum which originally .separated the branchial from the oral 

 chamber. 



Comparison of the Oral Chaiiibcr of Aintiiocales ivi/h that of 

 Eurypterus. Meamng of the Vlh Nei-oe. 



Passing now to the oral chamber — i.e. to the visceral struc- 

 tures innervated by the Vth nerve — we find, as already 

 suggested, distinct evidence in Ammoccetes of the presence of 

 the modified prosomatic appendages of the original Eurypterus- 

 like form. The large velar appendage is the least modified, 

 possessing as it does the arthropod tubular tnuscles, a blood 

 system of lacunar blood-spaces, and a surface covered with a 

 regular scale-like pattern, formed by cuticular nodo.sites, similar 

 to that found on the surface of Eurypterus and other scorpions. 

 The velar appendages show, further, that they are serially 

 homologous with the respiratory appendages, in that they have 

 been utilised to assist in respiration, their moveiuents being 

 synchronous with the respiratory movements. 



The separate part of the Vth nerve which supphes the velar 

 appendage passes within it from the dorsal to the ventral part 

 of the animal, and then, as Miss Alco^k has shown, turns 

 abruptly forward to supply the large median tentacle. This 

 extraordinary course leads directly to the conclusion that this 

 median tentacle, which is in reality double, constitutes, with the 

 velum of each side, the true velar appendages- 



Again, on each side of the middle line there are in Ammo- 

 ciEtes four large tentacles, each of which possesses a system of 

 muscles, muco-cartilage, and blood-spaces, precisely similar to 

 the median ventral tentacle already mentioned. Elach of these 

 is supplied, as Miss Alcock has shown, by a separate branch ot 

 the motor part of the Vth nerve (see Fig. 6), and each branch 

 is comparable with the branch supplying the large velar 

 appendage. 



That such tentacles are not mere sensory papillre surrounding 

 the mouth, but have a distinct and important morphological 

 meaning, is shown by the fact that they are transformed in the 

 adult Petromyzon into the remarkable tongue and suctorial 

 apparatus : a modification of oral appendages into a suctorial 

 apparatus which is abundantly common among Arthropods. 



Finally, the Vth nerve innervates the visceral muscles of the 

 lower and upper lips of Ammoccetes. In order, then, for the 

 story to be complete, the homologues of the lower and upper 

 lips must also be found in the system of prosomatic appendages 

 of forms like Limulus and Eurypterus. The lower lip, like the 

 opercular or thyroid appendage, po.ssesses a plate of muco- 

 cartilage, and, as already mentioned, falls into its natural place 

 as the metastoma of the old Euryplerus-like form, by the 

 enlargement and forward growth of which the oral chaniber 01 

 .\mmoco:tes was formed. The meaning of the upper lip will 



