84 GUIDE TO THE FOSSIL INVERTEBEATE ANIMALS. 
Gallery 
VIII. 
Table-case 
26. 
Table-case 
25. 
the anus opens, bears a pair of ai)pendages, attaclied to 
transverse thickenings of tlie ventral membrane. The front 
pair form wliip-like antennae. The remaining ]jairs are 
branclied, one branch being a crawling leg, the other branch 
bearing a fringe of bristles or of lamellae. The basal seg- 
ments of the four pairs on the head served to bite food and 
to pass it into the month. The lamellate branches of the 
remaining limbs may have served partly for swimming, 
partly for breathing. 
Trilobites lived only in the sea, some on reefs, some on 
mnddy or sandy bottoms; some, it is inferred either from 
the absence or the extraordinary size of the eyes, in deep 
Pig. 39. — Reconstruction of a Trilobite, Triarthncs Becki, from the 
Ordovician, Utica Slate of New York; a, upper side; b, under side; 
slightly^enlarged. (After Beecher. Table-case 25.) 
■water. In the growth of an individual trilobite of simple 
structure, the free cheeks and the eyes borne by them are 
at first not seen on the upper surface of the head-shield. As 
the animal grows they appear at the edge, and gradually 
come to occupy more and more of the upper surface. Some 
early trilobites, however, such as Agnostus (Fig. 40 a), 
Harpes, and Trinudem, never reach this stage, and may be 
separated as a Grade Hypoparia (under-cheeks) from those 
in which the free cheeks are visible on the up})er surface. 
In these latter the free cheeks may be confined to the fore- 
part of the shield, as in Calymmene, Staurocephalm (Fig. 40 c), 
and Pliacops (Fig. 38), or they may stretch right back so as 
