LUNG-BOOK OF SCOKPIO AND GILL-BOOK OF LIMULUS. 341 
ration the exact condition of the modern Scorpion’s lung- 
book. The appendages growing thus inwards by introversion 
(instead of outwards, as is normal) would simply be tucked or 
pushed into the great blood-sinus, which would constitute 
around each in-grown appendage a veinous sac, just as we 
actually find in the Scorpion. The most familiar case of 
inward growth taking the place of outward growth is in the 
development of the Tsenia-head upon the cyst of the hydatid 
in such a form as T. solium. The head develops in a per- 
fectly normal way, excepting that it is completely introverted, 
pushed outside in, and at a certain stage it becomes everted, as 
it should have been from the first, bad it retained in growth 
its ancestral relations. The cause of the introverted growth of 
the Tsenia-head on its cyst is very probably external pressure ; 
in fact, the growing mass of tissue takes the direction of least 
resistance, and grows into the cyst instead of out from it. 
It is not at all improbable that such a condition of external 
pressure might in the first instance have induced the inward 
growth, during development, of the lung-books of the Scor- 
pion. The development of the young Scorpion goes on at the 
present day under very remarkable conditions, actually in the 
ovary, the egg-cell never moving from its place of origin until 
it has grown into the fully-formed Scorpion ; the pressure of 
the ovarian tunic upon the surface of the growing embryo must 
he considerable, and is at any rate a possible cause of the in- 
vagination of the four hindermost pairs of mesosomatic appen- 
dages in the first instance. Probably the lamelligerous appen- 
dages of the young Scorpions, of a certain stage in the ancestry 
of recent Scorpions, were everted and assumed the normal 
relations of appendages as external processes of the body-wall 
as soon as the young were born. But as the lamelligerous 
appendages were only required to act as aerial respiratory 
organs it would be no disadvantage, but positively an advan- 
tage, that they should remain in the introverted condition; and 
this at last has become the permanent condition. This hypo- 
thesis accounts for the fact that the four pairs of lung-books 
do not ever appear on the surface of the embryo Scorpion as 
