144 THE APPENDAGES, ANATOMY, AND RELATIONS OF TRILOBITES. 



cation is the temporary suppression of the posterior pairs of appendages of the head, so that 

 they are generally developed later than the thoracic limbs. The median or nauplius eye 

 has not yet been found in trilobites, and if it is, as it appears to be, a specialized eye, it 

 has probably arisen since the later Crustacea passed the trilobite stage in their phylogeny. 



The oldest Crustacea, other than trilobites, so far known are the Branchiopoda and 

 Phyllocarida described by Walcott and discussed above. It is important to note that while 

 the former have already achieved such modified characteristics that they have been referred 

 to modem orders, they retain the trilobite-like limbs and some of them still have well devel- 

 oped pleural lobes. 



Caiman (1909, p. 101) says of the Copepoda: 



On the hypothesis that the nauplius represents the ancestral type of the Crustacea, the Eucopepoda would 

 be regarded as the most primitive existing members of the class, retaining as they do, naupliar characters in 

 the form of the first three pairs of appendages and in the absence of paired eyes and of a shell-fold. As 

 already indicated, however, it is much more probable that they are to be regarded as a specialized and in 

 some respects degenerate group which, while retaining, in some cases, a very primitive structure of the 

 cephalic appendages, has diverged from the ancestral stock in the reduction of the number of somites, the 

 loss of the paired eyes and the shell-fold, and the simplified form of the trunk-limbs. 



If the Eucopepoda be viewed in the light of the theory of descent here suggested, it 

 is at once seen that while they are modified and specialized, they more nearly approximate 

 the hypothetical ancestor than any other living Crustacea. Compound eyes are absent, and 

 it can not be proved that they were ever present, although Grobben is said to have observed 

 rudiments of them in the development of Calanus. The "simplified limbs" are the simple 

 limbs of the trilobite, somewhat modified. The absence of the shell-fold and carapace is 

 certainly a primitive characteristic. Add to this the direct development of the small number 

 of segments, and the infolded pleural lobes, and it must be admitted that the group pre- 

 sents more trilobite-like characteristics than any other. It seems very likely that the primi- 

 tive features were retained because of the pelagic habitat of a large part of the group. 



Ruedemann (Proc. Nat. Acad. Sci., vol. 4, 1918, p. 382, pi.) has recently outlined a 

 possible method of derivation of the acorn barnacles from the phyllocarids. Starting from 

 a recent Balanus with rostrum and carina separated by two pairs of lateralia, he traces back 

 through Calophragmus with three pairs of lateralia to Protobalanus of the Devonian with 

 five pairs. Still older is the newly discovered Eobalanus of the upper Ordovician, which 

 also has five pairs of lateralia but the middle pair is reversed, so that when the lateralia of 

 each side are fitted together, they form a pair of shields like those of Rhinocaris, separated 

 by the rostrum and carina, which are supposed to be homologous with the rostrum and 

 dorsal plate of the Phyllocarida. Ruedemann suggests that the ancestral phyllocarid attached 

 itself by the head, dorsal side downward, and the lateralia were developed from the two 

 valves 'of the carapace during its upward migration, to protect the ventral side exposed in 

 the new position. 



This theory is very ingenious, but has not been fully published at the time of writing, 

 and it seems very doubtful if it can be sustained. 



Summary. 



The salient points in the preceding discussion should be disentangled from their setting 

 and put forward in a brief summary. 



It is argued that the ancestral arthropod was a short and wide pelagic animal of few 

 segments, which so far changed its habits as to settle upon a sub-stratum. As a result of 



