No. 634] PHYLOGENY OF THE ARTHROPOD A 405 



blances are, however, in the appendages. All these, 

 except the antennules, maxillae, and maxillipeds, are bira- 

 mous, the antennae and mandibles being especially like 

 those of trilobites. The Copepoda are easily derivable 

 from the latter, even though there are no fossil forms to 

 connect the two groups. Since they lack compound eyes, 

 and show very slight evidence of having ever possessed 

 them, it is even conceivable that they branched off from 

 the Hypoparia, the most primitive of trilobites. 



The Ostracoda and Cirripedia are of course highly 

 modified by their somewhat peculiar method of life, but 

 when the young are studied, the characteristics of trilo- 

 bites are readily observed. In fact, it is really surpris- 

 ing that the trilobite-like character of the crustacean nau- 

 plius is so consistently ignored. It has a broad depressed 

 form like a trilobite. It has simple antennules, biramous 

 antennae, and mandibles like a trilobite, and the gnatho- 

 bases of the last two function as mouth-organs. An hy- 

 postoma is present, and there is a growing point, as is 

 evidenced by the way new segments are added. It is not, 

 it is true, a trilobite, but it looks like a trilobite modified 

 by suppression, and taken in connection with other evi- 

 dence, certainly does no injury to the theory that the 

 higher Crustacea were derived from the trilobites. 



The Malacostraca can be mentioned but briefly. Their 

 most ancient representative whose appendages are 

 know, Hymenocaris from the Middle Cambrian of Brit- 

 ish Columbia, had biramous appendages like those of the 

 trilobites, and most of the modern members of the group 

 have similar ones on some part of the body. As in lower 

 crustaceans, when the exopodites are lost or degenerate, 

 epipodites are developed to replace them, and thus the 

 limbs become variously modified. It is remarkable, how- 

 ever, how close a resemblance there is between the ap- 

 pendages of a trilobite and those of fresh-water syn- 

 carids from Tasmania, and even in the Decapoda the 

 seven segments of the walking leg are serially homolo- 

 gous with the seven segments of that of the trilobite. It 



