368 PROF. E. R. LANKESTER ON THE MUSCULAR AND 
taken on in the embryonic condition a very common trick of growth, viz. an inward 
growth of invagination, so that they grew into the Scorpion’s body, turning their 
outside in, just as a glove may have all its fingers and part of the hand turned outside 
in—then we should have without further alteration 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 7wnia-head upon the cyst of the 
hydatid in such a form as 7. solium. The head develops in a perfectly 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, had it retained in growth its 
ancestral relations. The cause of the introverted growth of the T@nia-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 Scorpion. 
The development of the young Scorpion goes on at the present day under very remark- 
able 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 be considerable, and is at any rate a 
possible cause of the invagination of the four hindermost pairs of mesosomatic appen- 
dages in the first instance. Probably the lamelligerous appendages 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 
advantage, that they should remain in the introverted condition; and this at last has 
become the permanent condition. ‘This hypothesis accounts for the fact that the four 
pairs of lung-books do not ever appear on the surface of the embryo Scorpion as 
up-standing appendages. ‘They are from the first introverted, and remain so. It also 
agrees with the disposition of the cuticularized surfaces of the Scorpion’s lung-book as 
seen in the adult. The cuticularized surface remains in the in-pushed as it is in the 
out-growing appendage, the surface in contact with the air. Each bag-like lamella 
is introverted together with the axis of the limb; and one cannot better picture to 
oneself the relative conditions of out-growth and in-growth than by fixing a kid glove 
by the margin of its opening to the margin of an opening of the same size on the 
outside of a box. The coloured surface of the kid will represent the cuticle, the 
fingers the lamelle, the hand the axis. Thus the glove will represent a lamelligerous 
appendage, standing up on the ventral surface of an Arthropod, its cavity communi- 
