146 THE APPENDAGES, ANATOMY, AND RELATIONS OF TKILOBITKS. 



origin. In this line of development two pairs of tactile antenn;c were produced, while the 

 anomomeristic character of the trilobite was retained. From similar opisthoparian ances- 

 tors there were, however, derived primitive Malacostraca retaining biramous antenme, 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 frorq 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 (Cydopygc), the entire cheek is broken up into lenses. Since a free 

 cheek is a separate entity, it is conceivable that it might lie modified into a movable organ. 



EVOLUTION OF THE MEROSTOMATA. 



It has been pointed out above that the Limulava (Sidncyia, Amiella, Emeraldclla) 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 antennce. 

 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 Sidncyia, and probably not 

 in Emcraldella, 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 chuetopod 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 (1911), they did not have the advantage 

 of studying the wonderful series of Crustacea described by him in 1912. While the evi- 



