TRANSACTIONS OF SECTION I, 957 
vertebrate skeleton all show how the bony skeleton is formed from the cartilaginous, 
and how the cartilaginous skeleton can be traced back to that found in Petromyzon, 
and so to the still simpler form found in Ammoccetes ; 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 modifications of the chitinous ingrowths of Limulus 
was suggested by Gegenbauer! so long ago as 1858, in consideration of the near 
chemical resemblances between the chitin and mucin groups of substances. 
Comparison of the Thyroid and Hyo-branchial Appendage of Ammocetes 
with the Opercular Appendage of Eurypterus, Thelyphonus, ée. 
Meaning of the VIIth Nerve. 
Seeing, then, how easily the IXth and Xth nerves in Ammoccetes correspond 
to the mesosomatic nerves to the branchial appendages in Limulus, and therefore 
to the corresponding nerves in such an animal as Eurypterus, we may with con- 
fidence proceed to the consideration of the VIIth nerve, and anticipate that it will 
be found to innervate a mesosomatic appendage in front of the branchial appendages, 
and yet belonging to the branchial group; in other words, if the VIIth nerve is to 
fit into the scheme, it ought to innervate a structure or structures corresponding 
to the operculum of Limulus or of Thelyphonus, &c. Now we see in figs. 5 and8 
the nature of the operculum in Eurypterus and in Thelyphonus, Phrynus, &c. 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 branchie ; 
so that the operculum 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 immediately in front of the glosso-pharyngeal seg- 
ment in Ammoceetes (fig. 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 Kurypterus and Ammoccetes 
(of. 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 VIIth nerve; so that every 
argument which has forced us to the conclusion that the elosso-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 con- 
sisted of two parts, a branchial and a genital, probably indicating the fusion of 
two segments ; and that the thyroid gland belonged to the genital operculum, just as 
the branchize belonged to the branchial operculum. This interpretation 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 still 
going on as to whether the facial supplies two segments or one. 
What, then, is the thyroid gland? Of all the organs found 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, 
important 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 Ammoceetes, to that larval condition so pregnant with inver- 
tebrate surprises, we find that the thyroid has become a large and important organ, 
1 Gegenbauer, ‘ Anat. Untersuch. eines Limulus,’ Abhandl. dex Naturf. Gesclisey. 
an Halle, 1858. 
