COMPARISON OF APPENDAGES. 69 



HOMOLOGY OF THE CEPHALIC APPENDAGES WITH THOSE OF OTHER CRUSTACEA. 



The head of the typical crustacean bears five pairs of appendages, namely, the antennules, 

 antennas, mandibles, and first and second maxillae, or, as they are more properly called, the 

 maxillulae and maxillae. 



As Beecher has pointed out, the "antenme" of the trilobites, on account of their pre- 

 oral position and invariably uniramous character, are quite certainly to be correlated with 

 the antennules. 



The second pair of appendages, the first pair of biramous ones, Beecher homologized 

 with the antenna; of other crustaceans, and that homology has been generally accepted, 

 though Kingsley (1897) suggested that it was possible that no representatives of the true 

 antennae were present. 



In preparing the restorations in the present study, the greatest difficulty has been to 

 adjust the organs about the mouth. In Triarthrus, numerous specimens show that without 

 question there are four pairs of gnathites back of the hypostoma, and that all four belong 

 to the cephalon. In forms with a long hypostoma, however, there was no room on the 

 cephalon for the attachment of four pairs of gnathites, neither were there enough appet> 

 difers to supply the requisite fulcra. At first I supposed I had solved the difficulty by 

 assuming the mouth to be in front of the posterior tip of the hypostoma, as it really is in 

 Ceraurus and Calyinenc, and allowing the gnathites to play under the hypostoma as Wal- 

 cott (1912) has shown that they do in Marrella. Finally, when I came to study in greater 

 detail the slices of Calymene and Ceraurus, they seemed to show that the anterior one or two 

 pairs of appendages became degenerate and under-developed. This was probably a special- 

 ization due to the great development of the hypostoma in trilobites, that organ being much 

 more prominent in this than in any other group. As the hypostoma lengthened to accom- 

 modate the increasing size of sub-glabellar organs (stomach, heart, etc.), the mouth mi- 

 grated backward, leaving the anterior appendages ahead of it, with their gnathobases, at 

 least, f unctionless. That such migration has taken place, even in Triarthrus, is shown by 

 the fact that the points of articulation of the first biramous appendages are pre-oral, and it 

 is more obviously true of Ceraurus. Correlated with the weakening of the appendages 

 on the lower surface is the loss of glabellar furrows on the upper surface. The glabellar 

 furrows mark lines of infolding of the test to form the appendifers and other rugosities for 

 the attachment of tendons and muscles. It is conceivable that this migration backward of 

 the mouth began very early in the history of the race, and that even before Cambrian times, 

 the antennae, probably originally biramous appendages like those on the remainder of the 

 body, had dwindled away and become lost. If this is the case, then the first pair of bira- 

 mous appendages of Triarthrus would be mandibles, the second pair maxillulne, and the third 

 pair maxillae. 



There remain the last pair of cephalic appendages, and they bring up the whole head 

 problem of the trilobites. Beecher has stated (1897 A, p. 96) his conviction that the head 

 of the trilobite is made up of five segments, representing the third, fourth, fifth, sixth, and 

 seventh neuromeres of the theoretical crustacean. As a matter of fact, he really made up 

 the head of seven segments, since he stated that the first neuromere was represented by the 

 hypostoma and the second by the epistoma and free cheeks. 



Jaekel (1901, p. 157) nearly agreed with Beecher, but made eight segments, as he saw 



