256 THE ORIGIN OF VERTEBRATES 
manner to the interpretation of all the organs in the head-region of 
the vertebrate from the corresponding organs of the arthropod. 
That it is possible to bring together all the striking resemblances 
between organs in the two classes of animals, such as I have done in 
preceding chapters, has been ascribed to a perverted ingenuity on my 
part—a suggestion which is flattering to my imaginative powers, but 
has no foundation of fact. There has been absolutely no ingenuity 
on my part; all I have done is to compare organs and their nerve- 
supply, as they actually exist in the two groups of animals, on the 
supposition that there has been no turning over on to the back, no 
reversal of dorsal and ventral surfaces. The comparison is there for 
all to read; it is all so simple, so self-evident that, given the one 
clue, the only ingenuity required is on the part of those who fail 
to see it. 
The great distinction that has arisen between the two head-regions 
is the disappearance of appendages as such, never, however, of 
important organs on those appendages. If the olfactory organs of 
the one group were originally situated on antennules, the olfactory 
organs still remain, although the antennules as such have disap- 
peared. The coxal excretory organs at the base of the endognaths 
remain and become the pituitary body. A special sense-organ, such 
as the flabellum of Limulus or the pecten of scorpion, remains and 
gives rise to the auditory organ. A special glandular organ, the 
uterus in the base of the operculum, remains, and gives rise to the 
thyroid gland. The branchiz and sense-organs on the mesosomatic 
appendages remain, and even the very muscles to a large extent. 
As will be seen later, the excretory organs at the base of the 
metasomatic appendages remain. It is merely the appendage as 
such which vanishes either by dwindling away, or by so great an 
alteration as no longer to be recognizable as an appendage. 
This dwindling process was already in full swing before the 
vertebrate stage ; it is only a continuation of a previous tendency, as 
is seen in the dwindling of the prosomatic appendages in the Mero- 
stomata and the inclusion of the branchie within the body of the 
scorpion. Already among the Palxostraca, swimming had largely 
taken the place of crawling. The whole gradual transformation from 
the arthropod to the vertebrate is associated with a transformation 
from a crawling to a swimming animal—with the concomitant loss 
of locomotor appendages as such, and the alteration of the shape of 
