NOTES ON THE EMBRYOLOGY OF LIMULUS. 
561 
together with the mandibular glands of Hexapods and the 
cheliceral glands of Spiders, may throw some light on this 
point. 
If, as pointed out by Lankester, the tracheae of the Arachnids 
have arisen from the gills of the Limulus, 1 then either those of 
the Hexapods must have been derived from those of the 
Arachnids, or those in the two groups must have arisen inde- 
pendently. The latter I believe to be the true case, and with- 
out entering into any argument to establish this point, I 
would call attention to the fact that in the terrestrial Isopods 
(Oniscidae) the gills become permeated with trachea-like air- 
tubes. Long ago Lereboullet (’ 52 , pi. vii, figs. 148 and 149) 
figured these ramifying tubes, while, to consult a more recent 
student, Leydig (’ 78 , p. 265, pi. xi, fig. 32) describes the 
mode of their formation. The cells of the blood-spaces of the 
gills secrete an internal cuticula, and in the branching cavities 
which this contains the air circulates. This forms tracheae, 
certainly without any phylogenetic connection with those of 
the so-called tracheates, but which present many analogies 
with them. It also affords good grounds for the supposition 
of Professor Lankester that the trachea, at least in some 
groups, have followed the course of pre-existent blood-vessels. 
Another fact of some weight in this connection lies in the 
position of the tracheal openings. In the Arachnids they 
1 Schimkewitsch ( 84 a ) denies this, but offers no reasons, but rather seems 
to misapprehend the whole argument relative to the relationships of Limulus to 
the Arachnids. He says (1. c., p. 67), “ Les poumons memes peuventetre con- 
siders comme une modification des trachees en faisceaux des chenilles et des 
myriapods. II est tres probable que les ancetres des Arachnides et des autres 
Tracheates avaient cette forme des trachees en faisceaux, laquelle a ete trans- 
formee cliez les Araignees en poumons. Tout cela me fait croire que les 
formes tracheennes et les Tetrapneumones sont plus anciennes que les 
Dipneumones.” Further on (p. 84) he says, “ Par leur apparail circulatoire et 
leur systeme musculaire, les Arachnides superieures se rapprochent au contraire 
des Limulides ; mais cette resemblance peut etre expliquee par l’identite 
qu’existe dans la configuration generale du corps de ces deux formes; car les 
Limules, d’apres leur evolution (etat Nauplius et etat de Trilobite) et d’apres 
la constitution des l’appariel respiratoire, sont des vrais Crustacea prives 
d’antennes.” 
