NOTES ON THE EMBRYOLOGY OF LIMULUS. 
565 
existence in Hexapods and Myriapods loses some of its force. 
If the reverse be true the argument, derived from these organs, 
for the maintenance of the group Tracheata has even less 
weight. 
From these and other facts it seems to me probable that the 
ancestral Hexapod left the main Arthropod stem some time 
before the separation of the Crustacea and Acerata. The 
common ancestor of all three was an elongate animal with 
flattened ambulatory appendages, some of which (except pos- 
sibly the first) were adapted for the purposes of eating. The 
head bore on its dorsal surface scattered sensory (optic) 
organs. In most, if not all, the segments of the body were 
segmental organs, through which passed the products of the 
genital glands, no genital ducts being differentiated. The 
reproductive glands were probably ventral in position ; the 
nervous system consisted of a single supra-oesophageal gan- 
glion, a ventral chain and a supra-intestinal loop (vide Peri- 
patus). The alimentary canal traversed the whole length of 
the body ; and, contrary to the opinions of the Hertwigs (’ 81 ), 
it seems probable that circulation was effected by the pulsation 
of the dorsal splanchnopleural muscles, the coelom containing 
the circulatory fluid, and that this portion was constricted off 
to form the heart. 
The Hexapods left this stem at a time when the first pair of 
originally post-oral appendages were moving towards a pre-oral 
position . 1 The Spiders and Crustacea continued together until 
they had the following characters in common : — One pair of 
appendages had a distinctly pre-oral position, and the basal 
joints of at least the two succeeding pairs were employed in 
the comminution of food. The segmental organs had dis- 
appeared from the first, third, fourth, and sixth body segments 
and possibly from others. The genital glands became con- 
centrated near the middle of the body, and the reproductive 
products passed out through the segmental organs in the same 
region. The genital glands themselves had assumed a dorsal 
1 Hatschek (’ 77 ) says that the ganglia of the mandible of the embryo 
Bombyx becomes transformed into part of the oesophageal connectives, 
von. XXV. NEW SER 
o o 
