NOTES ON THE EMBRYOLOGY OF LIMULUS. 
563 
aware, been seen by any other observer, and needs confirmation. 
Among other groups of Tracheates are the peculiar antennae of 
the Pauropids, with their basal joints bearing two branches, one 
of which in turn is bifid. The investigations of Mr. Wood-Mason 
(’ 79 ) show that in some of the Thysanura and in the Cock- 
roaches some of the appendages exhibit a biramose condition. 
While some of his facts and many of his speculations need con- 
firmation, 1 I think that in some instances (enough for our 
purposes) he has proved his points. Probably more extended 
studies on the Thysanura will throw important light on the 
relationships of the Hexapods to other forms. Patton (’ 84 , 
p.596) describes a similar condition in Blatt a germanica: 
“ A rather striking variation was found in the first and second 
maxillae of Blatta, which were formed respectively of two and 
three branches, the second maxillse thus attaining the typical 
trichotomous structure of the Crustacean appendages.” 
The existence of Malpighian tubes in Spiders and Hexapods 
is, without doubt, the greatest obstacle to the arrangement of 
the Arthropods here advocated, since they are absent in Limulus 
and the typical Crustacea. Their mode of development debars 
us from considering them as modified segmental organs, and we 
can only say that they are either inherited from an ancestor 
common to all Arthropods and have disappeared from some 
groups, or we must admit that they have appeared indepen- 
dently in two or more branches of the Arthropods. 'Which of 
these two alternatives we must take cannot be at present de- 
cided, though some facts may throw light upon the subject. 
The first thing that we notice is that those groups (Hexa- 
pods, Myriapods, and Arachnids) which possess them are ter- 
restrial, while the great bulk of the Crustacea are aquatic ; and 
this at once suggests that the organs may have an origin from 
similar physiological causes without any phylogenetic connec- 
tion existing between them. Turning to those Crustacea which 
lead a terrestrial life we meet with some striking confirmations 
of this hypothesis. 
Zenker (' 54 , pp. 106, 107) describes inlthe young Asellus 
1 E.g The structure of the mandible in Machilis. 
