FUNCTION OF APPENDAGES. 7 1 



they were also used in swimming is of course possible, but that was not their chief function. 

 It should be remembered that the exopodites are always found dorsal to or above the endopo- 

 dites, and in a horizontal plane. For use in swimming it would have been necessary to 

 rotate each exopodite into a plane approximately perpendicular to or at least making a con- 

 siderable angle with the dorsal test. In this position, the exopodites would have been thrust 

 down between the endopodites, and one would expect to find some specimens in which a 

 part at least of the exopodites were ventral to the endopodites. Specimens in this condi- 

 tion have not yet been seen among the fossils. To avoid having the exopodites and endopo- 

 dites intermingled in this way, the animal would have to bring all the endopodites together 

 along the axial line in a plane approximately perpendicular to the dorsal test, in which case 

 the exopodites would be free to act as swimming organs. The fact that the setae of an 

 exopodite stay together like the barbs on a feather would of course tend to strengthen the 

 idea that the exopodites could be used in swimming, but that is not the only possible ex- 

 planation of this condition. The union of the basipodite and exopodite shows that the two 

 branches of the appendage acted together. Every movement of one affected the other, and 

 the motion of the endopodites in either swimming or crawling produced a movement of the 

 exopodites which helped to keep up a circulation of water, thus insuring a constant supply 

 of oxygen. 



Although Neolenus is usually accounted a less primitive form than Ptychoparia or 

 Triarthrns, it has much the most primitive type of exopodite yet known. It would appear 

 that the exopodites were originally broad, thin, simple lamellae, which became broken up, 

 on the posterior side, into fine cylindrical setae. As development progressed, more and more 

 of the original lamella was broken up until there remained only the anterior margin, which 

 became thickened and strengthened to support the delicate filaments. The setae in turn be- 

 came modified from their original simple cylindrical shape to form the wide, thin, blade-like 

 filaments of Cryptolithus and Ccraurus. 



Another possible use of the exopodites is suggested by the action of some of the bar- 

 nacles, which use similar organs as nets in gathering food and the endopodites as rakes 

 which take off the particles and convey them to the mouth. The exopodites of the trilo- 

 bite might well set up currents which would direct food into the median groove, where it 

 could be carried forward to the mouth. 



Endopodites. 



The endopodites were undoubtedly used for crawling; in some trilobites, probably most 

 of them, for swimming; in the case of Cryptolithus, and probably others, for burrowing; and 

 probably in all for gathering food, in which function the numerous spines with which they 

 are arrayed doubtless assisted. 



Various trails have been ascribed to the action of trilobites, and many of them doubtless 

 were made by those animals (see especially Walcott, 1918). Some of these trails seem to 

 indicate that in crawling the animal rested on the greater part of each endopodite, while 

 others, notably the Protichnites recently interpreted by Walcott (1912 B, p. 275, pi. 47), 

 seem to have touched only the spinous tips of the dactylopodites to the substratum. The 

 question of the tracks, trails, and burrows which have been ascribed to trilobites is dis- 

 cussed briefly on a later page; but can not be taken up fully, as it would require another 

 monograph to treat of them satisfactorily. 



The flattened, more or less triangular segments of the endopodites of the posterior part 



