146 THE APPENDAGES, ANATOMY, AND RELATIONS OF TRILOBITES. 
origin. In this line of development two pairs of tactile antennas were produced, while the 
anomomeristic character of the trilobite was retained. From similar opisthoparian ances- 
tors there were, however, derived primitive Malacostraca retaining biramous antennae, but 
with a carapace and reduced pleural lobes and pygidium. From this offshoot were prob- 
ably derived the Ostracoda, the Cirripedia, and the various orders of the Malacostraca, with 
the possible exception of the Isopoda. I have suggested independent origins of the Copepoda 
and Isopoda, but realize the weighty arguments which can be adduced against such an 
interpretation. 
It is customary to speak of the Crustacea and Trilobita as having had a common ances- 
try, rather than the former being in direct line of descent from the latter, but when it can 
be shown that the higher Crustacea are all derivable from the Trilobita, and that they possess 
no characteristics which need have been inherited from any other source than that group, 
it seems needless to postulate the evolution of the same organs along two lines of develop- 
ment. 
I can not go into the question of which are more primitive, sessile or stalked eyes, but 
considering the various types found among the trilobites, one can but feel that the stalked 
eyes are not the most simple. While no trilobite had movable stalked eyes, it is possible 
to homologize free cheeks with such structures. They always bear the visual surface, and, 
in certain trilobites (Cyclopyge), the entire cheek is broken up into lenses. Since a free 
cheek is a separate entity, it is conceivable that it might be modified into a movable organ. 
EVOLUTION OF THE MEROSTOMATA. 
It has been pointed out above that the Limulava (Sidneyia, Amiella, Emeraldella) have 
certain characteristics in common with the trilobites on the one hand and the Eurypterida 
on the other. These relationships have been emphasized by Walcott, who derives the Euryp- 
terida through the Limulava and the Aglaspina from the Trilobita. The Limulava may be 
derived from the Trilobita, but indicate a line somewhat different from that of the remain- 
der of the Crustacea. In this line the second cephalic appendages do not become antennae, 
and the axial lobe seems to broaden out, so that the pleural lobes become an integral part 
of the body. As in the modern Crustacea, the pygidium is reduced to the anal plate, and 
this grows out into a spine-like telson. 
From the Limulava to the Eurypterida is a long leap, and before it can be made without 
danger, many intermediate steps must be placed in position. The direct ancestor of the 
Eurypterida is certainly not to be seen in the highly specialized Sidneyia, and probably not 
in Emeraldella, but it might be sought in a related form with a few more segments. The 
few species now known do suggest the beginning of a grouping of appendages about the 
mouth, a suppression of appendages on the abdomen, and a development of gills on the 
thorax only. Further than that the route is uncertain. 
Clarke and Ruedemann, whose recent extensive studies give their opinion much weight, 
seem fully convinced that the Merostomata could not have been derived from the Trilobita, 
but are rather inclined to agree with Bernard that the arachnids and the crustaceans were 
derived independently from similar chaetopod annelids (1912, p. 148). 
The greater part of their work was, however, finished before 1910, and although they 
refer to Walcott's description of the Limulava (191 1), they did not have the advantage 
of studying the wonderful series of Crustacea described by him in 1912. While the evi- 
