No. 2.] THE EMBRYOLOGY OF LIMULUS. 253 



opposed to the close association of the Trilobites with Limulus. 

 The body of Limulus, it must be remembered, possesses an 

 anterior cephalothorax bearing six pairs of circumoral chelate 

 appendages without differentiation into exopodite and endo- 

 podite,^ and with no trace of gills. In the Trilobite but four 

 pairs of appendages occur in this region. Hence, if the 

 "head" of the Trilobite is to be compared with the cephalo- 

 th.orax of Limulus we must assume — for which we have as yet 

 no evidence — that two pairs of appendages have been lost 

 from the Trilobite. The abdomen of Limulus bears six oairs of 

 broad leaf-like appendages, the posterior five pairs having 

 lamillate gill books upon the posterior surface. In the corre- 

 sponding region of the Trilobite, the thorax, we have an indefi- 

 nite number of somites, each of which bears the typical 

 Crustacean foot, consisting of basiopodite, exopodite, and en- 

 dopodite, and, outside the exopodite, occupying the same posi- 

 tion as the gill in the Decapod, a straight or curiously coiled 

 structure interpreted by Walcott as the gill. In the horse- 

 shoe crab the abdominal region extends to the anus, behind 

 which comes the non-segmental tail. In the Trilobites the 

 thorax is followed by a segmented pygidium on which the 

 series of appendages^ is continued to the end, and there is no 

 evidence of a supra-anal telson. 



The necessary conclusion is that the appendages of the Trilo- 

 bite vary in number and differ totally in structure from those of 

 Limulus, and the association of the Trilobites with the Xipho- 

 sura is not warranted by our present state of knowledge. The 

 trilobites would appear to be true Crustacea, the sessile eyes 

 and general shape of the body allying them to the Isopods, 



1 The flabellum of the sixth appendage cannot be considered as a representative 

 of the expodite since it develops later than the rest of the limb, develops inde- 

 pendently of it and only in the later embryonic stages does the base of the leg 

 enlarge so that it is included. 



2 Walcott continues the series of ambulatory appendages through thi-s region, 

 but Professor Mickleborough ('83), who found the specimen forming the basis 

 of Walcott's second paper ('84), thinks that these pygidial appendages were 

 lamellar. Henry Woodward ('70) describes what he considers as the jointed 

 palpus of one of the " maxillas " of Asaphus with seven articulations beyond the 

 basal joint. 



