644: PROFESSOR E. RAY LANKESTER, 
soma and six free segments to its metasoma—the metasoma 
devoid of appendages as in Xiphosura and Eurypterina, the 
mesosoma provided with a genital operculum (united ap- 
pendages) on its anterior segment and with five pairs of 
lamelligerous respiratory appendages on the five succeeding 
segments—from such a form by a very slight process of 
change, consisting in adaptation to terrestrial in place of 
aquatic conditions, the primitive Scorpions were developed. 
It is probable that the particular form antecedent to the 
differentiation of Xiphosura and Kurypterina, from which 
the Scorpions took origin had not developed lateral com- 
pound eyes, but still exhibited a primitive condition, which 
is retained by the Scorpions and other Arachnida, viz. a 
lateral grouping of simple eyes. 
The structural changes necessary to produce a Scorpion 
from such an ancestral Merostom as has been just sketched 
are so small that it is not possible to place the Scorpions 
and the Merostomata in separate classes, if by the use of the 
division known as a ‘class’ we are to indicate as nearly as 
possible, in different parts of the pedigree of animals, an 
equal break or unrepresented interval of structural change. 
At the same time the Scorpions, having once been developed, 
appear to have given rise to the whole series of living Arach- 
nida, to the Pedipalpi first, and through these to the 
Araneina, and through the Araneina to the Acarina. 
Galeodes is probably a special development from the 
Scorpionina, as in a different direction are the Opilionina 
and Pseudoscorpions. 
This conclusion, if it be well founded, justifies some im- 
portant inferences of a secondary character. In the first 
place we have to admit a very extensive process of degenera- 
tion in the course of development, leading from the Scorpion 
to such Acarina as Demodex, or even Hydrachna. In the 
second place we obtain a definite answer as to the mode of 
origin of trachez, in so far, at least, as the trachez of the 
Arachnida are concerned. The vascular lamelligerous 
appendages of the Limuloid ancester of the Scorpion became 
dry and filled with air in place of with blood. From this 
blood-sinus, converted into an air-sinus, the air appears gra- 
dually to have made its way, encroaching upon pre-existing 
blood-canals, and converting them into air-canals. The highly 
developed condition of the blood-vascular system in the 
Scorpions renders it probable that the trachee of the tra- 
cheate Arachnida are not mew vessels specially developed as 
an aérial vascular system, but are the modified and adapted 
blood-vascular channels, just in the same way as the air- 
