558 



NATURE 



[October 8, 1896 



Ammocretes. Another distinction is manifest between these 

 branchial cartilages and those of the trabecuhi; and auditory 

 capsules, in that the latter do not stain in the same manner ; 

 whereas the matrix of the branchial cartilages stains red with 

 picro-carmine, that of the trabecule and auditory capsules stains 

 deep yellow, so that the junction between the trabecula; and the 

 first branchial bar is well marked by the transition from the one 

 to the other kind of staining. The difference corresponds 

 to Parker's (Phil. Trans. Roy. Soc, 1S83) soft and hard 

 cartilage. 



The new cartilages which are formed at transformation, either 

 in places where muco-cartilage exists before or by the invasion 

 of the fibrous tissue of the brain-case by chondroblasts, are all of 

 the hard cartilage variety. 



The phylogenetic, anatomical, and ontogenetic history of the 

 formation of the vertebrate skeleton all show how the bony 

 skeleton is formed from the cartilaginous, and how the cartilag- 

 inous skeleton can be traced back to that found in Petromyzon, 

 and so to the still simpler form found in Ammoctetes ; from this, 

 again, we can pass directly to the cartilaginous skeleton of 

 Limulus, and so finally trace back the cranial skeleton of the 

 vertebrate to its commencement in the modified chitinous 

 ingrowths connected with the entapophyses of Limulus. A 

 similar explanation of the origin of cartilage from modifica- 

 tions of the chitinous ingrowths of Limulus was suggested by 

 Gegenbauer (" Anat. Untersuch. eines Limulus," Ahliandl. der 

 Naturf. Gesc/isch. in Halle) so long ago as 1858, in considera- 

 tion of the near chemical resemblances between the chitin and 

 mucin groups of substances. 



Comparison of the Thyroid and Hyci-hramhial Appendage of 

 Aminocaies with the Opercular Appendage of Euryp/eriis, 

 The'yphonus, &^e. Meaning of the Vllth Nerz'e. 



Seeing, then, how easily the IXth and Xth nerves in Am- 

 mocoetes correspond to the niesosomatic nerves to the branchial 

 appendages in Limulus, and therefore to the corresponding 

 nerves in such an animal as Eurypterus, we may with confidence 

 proceed to the consideration of the Vllth nerve, and anticipate 

 that it will be found to innervate a niesosomatic appendage in 

 front of the branchial appendages, and yet belonging to the 

 branchial group ; in other words, if the Vllth nerve is to fit 

 into the scheme, it ought to innervate a structure or structures 

 corresponding to the operculum of Limulus or of Thelyphoniis, 

 &c. Now we see in Figs. 5 and 8 the nature of the operculum 

 in Eurypterus and in Thelyphonus, Phrynus, cSic. It is in 

 reality composed of two parts, a median and anterior portion 

 which bears on its under surface the external genital organs, 

 and a posterior part which bears branchiae ; so that the oper- 

 culum of these animals may be considered as a genital operculum 

 fused to a branchial appendage, and therefore double. It is 

 absolutely startling to find that the branchial segment imme- 

 diately in front of the glosso-pharyngeal segment in Ammoccetes 

 (F'g' 3) consists of two parts, of which the posterior, the 

 hyo-branchial, is gill-bearing, while the anterior carries on its 

 under surface the pseudo-branchial groove of Dohrn, which 

 continues as a ciliated groove up to the opening of the thyroid 

 gland. 



Again, the comparison of the ventral surfaces of Eurypterus 

 and Ammocoetes (cf. Fig. 8) brings to light a complete 

 coincidence of position between the median tongue of the 

 operculum in the one animal and the median plate of muco- 

 cartilage in the other animal, which separates in so remarkable 

 a manner the cartilaginous basket-work of each side, and bears 

 on its under surface the thyroid gland. Finally, Miss Alcock 

 has shown that not only the hyo-branchial, but also the thyroid 

 part of this segment, is innervated by the Vllth nerve ; so that 

 every argument which has forced us to the conclusion that the 

 glosso-pharyngeal and vagus nerves are the nerves which 

 originally supplied branchial appendages equally points to the 

 conclusion that the facial nerve originally supplied the opercular 

 appendage— an appendage which closed the branchial chamber 

 in front, which consisted of two parts, a Ijranchial and a genital, 

 probably indicating the fusion of two segments : and that the 

 thyroid gland belonged to the genital operculum, just as the 

 branchi.e belonged to the branchial operculum. This inter- 

 pretation of the parts supplied by the facial nerve immediately 

 explains why Dohrn is so anxious to make a thyroid segment 

 in front of the branchial segments, and why a controversy is 

 slill going on as to whether the facial supplies two segments 

 or one. 



NO. 1406, \0L. 54] 



What, then, is the thyroid gland ? Of all the organs fouml 

 in the vertebrate, with perhaps the single exception of the 

 pineal eye, there is no one which so clearly is a relic of the 

 invertebrate ancestor as the thyroid gland. This gland, im- 

 portant as it is known to be in the higher vertebrates, remains 

 of much the same type of structure down to the fishes, and 

 even to Petromyzon ; suddenly, when we pass to the Am- 

 moccetes, to that larval condition so pregnant with invertebrate 

 surprises, we find that the thyroid has become a large and 

 important organ, totally different in structure from the thyroid 

 of all other vertebrates, though resembling the endostyl of the 

 Tunicates. 



The thyroid of Ammocretes may be described as a long tube, 

 curled up at its posterior end, which contains in its wall, along 

 the whole of its length, a peculiar glandular structure, confined 

 to a small portion of its wall. 



A section through this tube is given in Fig. 7, and shows how 

 this glandular structure possesses no alveoli, no ducts, but con- 

 sists of a column of elongated cells arranged in a wedge-shaped 

 manner, the apex of the wedge being in the lumen of the tube ; 



ISranchial Muco- 



cartilage. cartilage. 



Thyroid (.Ammoccetes). 



Ope, 



Thyroid (Scorpion). 

 Fig. 7. 



each cell contains a spherical nucleus, situated al the very extreme 

 end of the cell, farthest away from the lumen of the tube. Such 

 a structure is different from that of any other vertebrate gland. 

 Its secretion is not in any way evident. It certainly does not 

 .secrete mucus or take part in digestion, and for a long time I 

 was unable to find any structure which resembled it in the least 

 degree, apart, of course, from the endostyl of the Tunicates. 



Guideil, however, by the considerations already put forward, 

 and feeling therefore convinced that in Eurypterus there imist 

 have been a structure resembling the thyroid gland underneath 

 the median projection of the operculum, I proceeded to investi- 

 gate the nature of the terminal genital apparatus underlying the 

 operculum in the different members of the scorpion family, and 

 reproduce here (Fig. 8) the figures given by Blanchard 

 (" L'Organisation du Regne Animal'') of the appearance of the 

 terminal male genital organs in Phrynus and Thelyphonus. 

 Emboldened by the striking appearance of these figures, I pro- 

 ceeded to cut sections through the operculum of the European 

 scorpion, and found that that part of the genital duct which 

 underlies the operculum, and that part only, contains within its 



