TRILOBITES — WHITTINGTON 



409 



Number of subfamilies 

 in each series: 



I o O new 



n + • • •+ dying out 



Figure 2. — Evolutionary history of trilobites portrayed in the rise and fall of subfamily 

 groups in time. (From C. J. Stubblefield, 1959.) 



then extended back beneath the middle part of the body, terminating 

 in an anus at the posterior tip. St0rmer's reconstruction of the under- 

 side of the body shows an enclosing membrane and a pair of similar 

 appendages on each segment. Each appendage consists of a jointed 

 walking leg with bristles at the tip. From near the base of the ap- 

 pendage rises a jointed branch that bears many fine filaments. 



All investigations have shown that the trilobite's appendages were 

 similar on each segment, and that none bore a claw or pincer for grasp- 

 ing and tearing food and passing it to the mouth. Trilobites probably 

 fed, therefore, on minute organic particles suspended in the water or 

 enclosed in the sediment of the sea bottom, this material being brought 

 to the mouth by currents of water. The filament-bearing branches of 

 the appendages may have been the main instruments in producing 

 these currents. They probably also functioned as gills, and constant 

 movement of the branches would have kept the gills bathed with fresh 

 water. 



The trilobite's appendages were attached by muscles to the convex 

 middle region of the exoskeleton. Deep furrows in this region on the 

 head, thorax, and tail formed projections on the inside of the shell for 

 such attachments. Trilobites with smooth shells may show dark 

 patches, which are believed to be corresponding areas of muscle at- 

 tachment. The animal must also have possessed longitudinal muscles 

 to effect its characteristic enrollment : these were probably situated in 

 the middle region of the body. 



It had been argued that trilobites like Isotelus (pi. 8) , with its wide 

 middle region and its relatively large tail, may have used a dov»nward 

 and forward stroke of the tail in swimming, as does the modern lob- 

 ster. The bodies of these two animals are not comparable, however : 



