386 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 fiabellum 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 branchia? 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 branchia3 within the body of the 

 scorpion. Already among the Pakeostraca, 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 



