FUNCTION OF APPENDAGES. J I 
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 seta; 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 
Triarthrus, 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 Ceraurus. 
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 
