The eyes may be supported upon prominences, but they are never carried on movable stalks (as they are in the existing lobsters and crabs); and in some of the Cambrian Trilobites, such as the little Agnosti (fig. 31 g), the animal was blind.  The lateral portions of the head-shield are usually separated from the central portion by a peculiar line of division (the so-called “facial suture”) on each side; but this is also wanting in some of the Cambrian species.  The backward angles of the head-shield, also, are often prolonged into spines, which sometimes reach a great length.  Following the head-shield behind, we have a portion of the body which is composed of movable segments or “body-rings,” and which is technically called the “thorax,” Ordinarily, this region is strongly trilobed, and each ring consists of a central convex portion, and of two flatter side-lobes.  The number of body-rings in the thorax is very variable (from two to twenty-six), but is fixed for the adult forms of each group of the Trilobites.  The young forms have much fewer rings than the full-grown ones; and it is curious to find that the Cambrian Trilobites very commonly have either a great many rings (as in Paradoxides, fig. 31, a), or else very few (as in Agnostus, fig. 31, g).  In some instances, the body-rings do not seem to have been so constructed as to allow of much movement, but in other cases this region of the body is so flexible that the animal possessed the power of rolling itself up completely, like a hedgehog; and many individuals have been permanently preserved as fossils in this defensive condition.  Finally, the body of the Trilobite was completed by a tail-shield (technically termed the “pygidium"), which varies much in size and form, and is composed of a greater or less number of rings, similar to those which form the thorax, but immovably amalgamated with one another (fig. 31, h).

The under surface of the body in the Trilobites appears to have been more or less entirely destitute of hard structures, with the exception of a well-developed upper lip, in the form of a plate attached to the inferior side of the head-shield in front.  There is no reason to doubt that the animal possessed legs; but these structures seem to have resembled those of many living Crustaceans in being quite soft and membranous.  This, at any rate, seems to have been generally the case; though structures which have been regarded as legs have been detected on the under surface of one of the larger species of Trilobites.  There is also, at present, no direct evidence that the Trilobites possessed the two pairs of jointed feelers ("antennae”) which are so characteristic of recent Crustaceans.