1885.] Embryology. | 725 
Limulus is much like that of the spider, as seen in the mode of 
origin of the mesoblastic segments and the probable origin of the 
hypoblastic cells. There is a superficial resemblance between the 
embryo of Limulus and of the spider, as may be seen by a com- 
parison of our Fig. 2 and Balfour’s Fig. 15. Without much 
doubt the Tracheata and Palzocarida, as well as Crustacea Neo- 
carida, branched off from a common ancestor, but the more im- 
portant morphological points show that the terrestrial, air-breath- 
ing tracheates were a much later branch of the arthropod tree 
than the marine branchiate Paleocarida and genuine Crustacea. 
Probably the Palzocarida (Limulus and other Merostomata, and 
Trilobita) were the earliest arthropods to appear; after them arose 
the Crustacea, perhaps at nearly the same time the Arachnida, 
and finally the Myriopoda and the winged insects. Without 
much doubt the earliest branchiate forms were our Protocyclus,' 
the ancestor of the Palzocarida; and a protonauplius form, the 
forerunner of the Crustacea; these were marine, perhaps branchi- 
ate organisms, with a few pairs of simple oar-like swimming 
appendages either around or just behind the mouth, and which 
were free-swimming or creeping forms; the Protocyclus was, 
perhaps, a solid oval creeping animal living at the bottom on mud 
or sand. The branchiz probably became first developed on the 
limbs of the free-swimming Protonauplii, as they needed, owing 
to their great rapidity of movement, the means of rapid aération | 
of the blood; while in the heavily molded less oxygen-consum- 
ing Protocyclus, the evolution of gills was somewhat postponed. 
The steps from Protocyclus to Agnostus was not a very long one. 
The oldest arthropods, notwithstanding the recent discovery of a 
Silurian scorpion, were trilobites. 
The following conclusions are drawn from a study of the stage 
of Limulus here figured. 
The fact that the embryo Limulus had at first no abdominal 
appendages (uropoda), whereas there are temporary ominal 
appendages in the tracheates, shows that Limulus in this impor- 
tant respect has little in common with the Arachnida, Myriopoda 
or Hexapoda. On the other hand in the embryo Crustacea the 
cephalic limbs are first indicated ; the nauplian limbs as we 
the zoéan appendages being cephalic ; the uropods not appearing 
until after the Crustacea leave the egg. 
These facts indicate that Limulus probably descended from a 
type in which there were cephalic appendages only, and no 
abdominal appendages. The absence of a serous membrane, of 
an amnion, and of procephalic lobes, of temporary embryonic 
abdominal appendages (at the stage above described) ; also of 
protozonites (seen in the early embryo of the scorpion and spider) 
tend to prove that the embryo of Limulus has little in common 
with that of Tracheata. 
1 See Development of Limulus, 1872, p. 
