NOTES ON THE EMBRYOLOGY OF LIMULUS. 
539 
more readily effected on account of a change of the medium of 
respiration. A gill needs either to project freely into the 
water, or to have some means of constantly changing the fluid 
which bathes it. An organ for aerial respiration, on the other 
hand, is not so restricted in its position, since the air is more 
fluid and more elastic, and hence more readily changed. 
Another advantage to the animal resulting from the change 
is that the oxygen is thus- brought nearer to the tissues 
requiring it. 
In fig. 20 we have a diagrammatic representation of the pul- 
monary sac of the Arachnids. The appendage ( ga .) has now 
become sunk in the body and the hole through which it passed 
is the stigma (stg.) The gill lamellae have entirely disappeared 
and the pulmonary ones (pi.) have taken their place. The 
process here described is different from that imagined by 
McLeod (' 82 ). It accords more with the development of the 
gills in Limulus, and avoids the necessity of union of the gill- 
laminae and the expansion of the sternum. 
Having derived the lungs of the Scorpion in this manner, but 
little needs to be said concerning the origin of the tracheae in 
the spiders. Many years ago Leuckart showed that the so- 
called lungs of the Arachnids were but modifications of the 
peculiar tracheae of the same group. This conclusion holds 
good to-day, and I would accept it in an inverted condition: 
The tracheae of the Arachnids are but modifications of the pul- 
monary organs existing in some of the group. To transform 
the lungs into the other type of organ but slight changes are 
necessary. A prolongation of one of the sac-like pulmonary 
lamellae towards the thorax gives the condition found in 
Argyroneta; a slight amount of branching produces the 
tracheal system of Zilla, and so on through forms like Thomisus 
until the most complicated condition is reached. McLeod’s 
observations are interesting in this connection. 
The existence of the so-called spiral threads in the tracheae 
of some of the Arachnids is to be explained on mechanical 
grounds. In some forms nothing of the sort is found and here 
the tracheae are flattened tubes. To prevent them from being 
