368 PEOP. E. E. LANKESTBE ON THE MUSCULAE 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 T«Mia-head upon the cyst of the 

 hydatid in such a form as T. 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(5B«/«-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 

 ringers the lamellae, the hand the axis. Thus the glove will represent a lamelligerous 

 appendage, standing up on the ventral surface of an Arthropod, its cavity communi- 



