148 PACKARD—CLASSIFICATION OF ARTHROPODA. [April 3, 
to several annulations,’’ which probably represent segments. 
From this we logically infer that in the protaspis of trilo- 
bites there were more than three pairs of head-appendages, and 
possibly two or three pairs of abdominal appendages. Now the 
larva of Limulus is hatched with two body-regions, of the same 
general shape as those of the trilobites, and it is also trilobed; the 
embryo, sometimes before hatching, with its thick spherical body, 
strongly recalls the protaspis stage of trilobites, and seems to justify 
the view that the freshly hatched larva of Limulus is a protaspis.’ 
In the protaspis-like fossil Cyclus, which seems to represent an 
ancestral type of Limuloids persisting into the Carboniferous 
Period, there are traces of head-appendages like those of the 
embryo Limulus. 
The metamorphosis of the Palzopoda is, then, incomplete; the 
limbs of the protaspis retain the form and functions of the larva, 
the adult simply differing in acquiring at successive molts additional 
trunk-segments, with their corresponding limbs. 
The eggs of Limulus as well as of Arachnida are large and not 
sO Numerous as in some Crustacea ; those of Limulus are laid in the 
sand. The eggs of trilobites are also large, spherical, and evidently, 
like those of Limulus, were deposited in the sand or sandy mud, as 
they occur separately from the trilobites themselves. 
The embryology of Limulus presents some unique features, and 
yet there is such a close resemblance to that of the scorpion that 
the embryology of the Arachnida, as I have freely acknowledged, 
affords very strong proofs of their relationship to and descent from 
merostomes. In the embryo of the scorpion and spiders there are 
six pairs of head- (cephalothoracic) appendages, and the mode of 
origin of the book-lungs of the scorpion and spiders seems to prove 
that they are derivatives of the exopodites of the abdominal limbs 
of Limulus. 
It results from what is now known of the structure of the Trilo- 
11 freely acknowledge that many years ago (1872) I supposed that the embryo 
Limulus passed through a nauplius, and that I called it a « subzoéa stage,” but this 
view was long since abandoned, as also my contention that Limulus was nearer 
the Crustacea than the Arachnida. It need hardly be added that while as pre- 
viously I cannot agree with the view that Limulus is an actual Arachnid, it has 
for some years, through the result of the work of Kingsley and Kishinyoue, been 
evident that the Merostomes are closely related to the Arachnida, and I adopted 
this view in my memoir on the brain of Limulus (1893). 
