38 



Fis. 13. 



Thus, in tlie existing representative of Xiphosura, the embryo or larva is neither a 

 Nauplms nor a Zom, nor a Trilobite : it is a Limtihis, exhibiting the characters of such in 

 stages of development or growth corresponding to the period of incubation at which the 

 immature creature may be examined. 



§ 11. Conclusion. — That the Trilobite, like the Limulus, possessed articulate limbs, 

 has, however, been advocated not only by interpretation of appearances 

 in an exceptional instance, but by appeal to the laws of coexistence *. 

 I would submit, however, the following remarks bearing upon the cor- 

 relation of vision with other ways and means of locomotion. 



Limulus possesses the pair of relatively large compound eyes, set high 

 upon the lateral parts of the cephaletral carapace ; and besides these, it 

 has the pair of small anterior simple eyes : it looks forward and upward, 

 and commands, like the guns of a demilune bastion, a like range in the 

 horizontal sweep. And yet the prevalent impression, from the position 

 and proportions of its subcylindrieal jointed limbs, is that they subserve 

 the needs of digestion much more than those of locomotion. No observer 

 has yet testified to their capacity of uplifting the body from the ground, 

 whether dry or submerged, and of bearing it along by successive steps, 

 as do the jointed legs of the Isopod, the Lobster, or the Crab. Some of 

 the pairs are obviously incapable of such locomotive functions. The last 

 pair (vii. in all the figures) may help to push the body along the sand, 

 as the oar serves to shove off a boat ; but that is all. 



Eurypterus (fig. 13) and Pterygotus (fig. 14) possess, like Limulus, both the antei'o- 

 median ocelli and the medio-lateral compound eyes. In Tienjgotus the antennse are 



forcipated members for prehension of food, as in Limulus. The 

 three succeeding pairs of cephaletral limbs are still less capacitated, 

 through their proportionally smaller size and concomitant slender- 

 ness, for gradatorial movement of the body. They are adapted to 

 rout out of the sand or mud, disturbed by the spade-shaped head, 

 the objects of food which the front pair is modified to seize. The 

 larger terminal pair of limbs are more decidedly natatory in form 

 than are the last cephaletral lamelligerous pair (PI. II a, figs. 2, 4, vii) 

 iu Limulus. In JEurypterus the cephaletral limbs anterior to the 

 lamelliform natatory pair seem to be alike in structure, unless the 

 antennal forceps has been wanting in the fossils, and must have 

 had functions as limited as are their size and strength. 



These considerations weigh with me in checking a tendency to 



conclude that the Trilobites, because they had large compound eyes, 



Ptcnjgoitts awjUcus, Ag. must havc had articulate ambulatory limbs of as strong a texture. 



leri, Hbt. 



Fiff. 14. 



*■ "The large compouud sessile eyes, and the hard, shelly, many-segmented body, -svith its compound caudal and 

 head- shield, differ from any known Phyllopod, hut offer many points of analogy with the modern Isopods; and one 

 would be led to presuppose the Trilobites possessed of organs of locomotion of a stronger texture than mere bran- 

 chial frills." — H. Woodward, Geolorjkal Mnyazine, vol. viii. p. 523. 



