238 



INVEETEBEATA 



CHAP. 



similarly a median " glabellum," consisting of the fused terga of the 

 prosomatic region, and on each side of it a " fixed cheek," consisting 

 of the fused pleura of this region. Outside the fixed cheek there is a 

 marked suture or joint line, the " facial suture," which runs round 

 in a semicircle parallel with the edge of the carapace. The cephalic 

 lobe and its lateral extensions, which we may term " free cheeks," lie 

 beyond, and in these latter we find the compound lateral eyes (Fig. 188). 

 Point for point this structure is repeated in the cephalic shield of 

 a Trilobite, and so far the agreement between the Trilobite and the 

 larva is complete. But since the appendages of the Trilobite have 

 been made known to us by Beecher (1893) it is at once seen what a 

 wide divergence there is between them and those of Limulus. The 



Fig. 188. — The Trilobite larva of Limulus. (After Kingsley.) 



A, ventral view. B, dorsal view, /.c, free cheek ; f.s, facial suture ; fx.o, fixed cheek ; gl, glabellum ; 

 hep, lobes of liver seen through ; ^6^ the last walking leg ; op, genital operculum. 



Trilobite has unforked filiform antennae, and all the other appendages 

 are similar to one another. All possess a jointed endopodite with a 

 gnathobase, all have a rod-like exopodite which carries a " book " of 

 long narrow gills, essentially similar to those of Limulus, but the 

 contrast between the appendages of the prosoma and those of the 

 abdomen, which is so marked a feature in Limulus as in all true 

 Arachnida, is utterly absent in the Trilobite; moreover, Limulus 

 shows no trace of the antennae of the Trilobite. 



In a word the so-called Trilobite larva represents not a Trilobite 

 but an Arachnid, not very unlike the adult but with segments 

 which are quite free from one another, and without the long tail. 

 Such Arachnida are known to have existed in the Silurian epoch 

 {Hemiaspis), but the stock from which Arachnida and Trilobita 

 diverged must be still further back in the remote past. 



