This order is entirely extinct, none of its members having survived the close of the Palaeozoic period. The Trilobites are Crustaceans in which the body is usually more or less distinctly trilobed; there is a cephalic shield, usually bearing a pair of sessile compound eyes; the thoracic somites are movable upon one another, and are very variable in number; the abdominal segments are coalescent, and form a caudal shield; there is a well-developed upper lip or "hypostome."
As regards the general structure of the Trilobites, the body was protected by a well-developed shell or "crust," which covered the whole dorsal surface of the body, and which usually exhibits more or less markedly a division into three longitudinal lobes (fig. 148), from which the name of the order is derived. The crust is composed of a cephalic shield, generally crescentic in shape, a variable number of free and movable rings, constituting the thorax, and a caudal shield or "pygi-dium," the rings of which are more or less completely anchy-losed. On the under surface of the head-shield in front, there is situated a forked or oval upper lip or "labrum," which resembles in form the labrum of the Phyllopodous genus Apus. Recent researches by Mr C. D. Walcott have also considerably increased our knowledge of the condition of the under surface of the body in the Trilobites. This observer, namely, has shown that the visceral cavity of the Trilobites (fig. 149, b) was bounded inferiorly by a thin membrane, which is attached to the lower margin of the dorsal crust all round. This ventral membrane was strengthened by calcified arches, which in turn supported the appendages beneath. As to these latter our knowledge is not yet complete, but we know that in some forms there existed a row of articulated appendages on each side of the middle line below. The thoracic appendages seem to have been slender five-jointed legs, in which the terminal segment forms a pointed daw, and the basal segment carries a jointed appendage, regarded by Mr Walcott as homologous with the
Fig. 148. - The skeleton of a Trilobite (Angelina Sedgwickii), partially dissected. A, Head-shield. B, Movable rings of the thorax. C, Tail or abdomen. g Glabella, in this species without furrows; fi Fixed cheeks; e Eye - lobe; o Eye; f Facial suture; fr Free cheeks: s Head-spines; p Pleurae; p p Anchylosed pleurae of pygidium.
"epipodite" of many recent Crustaceans. On each side of the thoracic cavity there is, also, attached a row of bifid spiral appendages (fig. 149, e), of the nature of gills; and branchial appendages were probably attached to the bases of the thoracic limbs as well. With regard to the appendages of the head, the mouth is situated behind the hypostome, and is bounded by four pairs of jointed manducatory appendages, the basal joints of which are, partly or wholly, modified to act as jaws.
The cephalic shield of a typical Trilobite is more or less completely semicircular (fig. 148), and is composed of a central and of two lateral pieces, of which the two latter may, or may not, be united together in front of the former.
The median portion is usually elevated above the remainder of the cephalic shield, and is called the "glabella;" it protected the region of the stomach, and is usually divided into from three to four lobes by lateral grooves. At each side of the glabella, and continuous with it, is a small semicircular area, called the "fixed cheek." The glabella, with the " fixed cheeks," is separated from the lateral portions of the cephalic shield - termed the "movable" or "free cheeks" - by a peculiar suture or line of division, which is known as the "facial suture," and is quite unknown amongst recent Crustacea, except for a faint indication in the Limulus, and more or less doubtful traces in certain other forms. The movable cheeks bear the eyes, which are generally crescentic or reniform in shape, are rarely pedunculated (being never supported upon movable foot-stalks), and consist of an aggregation of facets covered by a thin cornea. The facial sutures may join one another in front of the glabella - in which case the free cheeks will form a single piece; or they may cut the anterior margin of the shield separately - in which case the free cheeks will be discontinuous. The posterior angles of the free cheeks are often produced into long spines.
Fig. 149. - Transverse section of the thorax of Calymene senaria, partially restored (after C. D. Walcott). a Dorsal crust; b Visceral cavity, continued laterally to the pleural margins of the dorsal crust; c Legs, restored; d Epipodite; e Spiral gills. Enlarged six times.
Behind the cephalic shield comes the thorax, composed of a variable number of segments, which are not soldered together, but are capable of free motion upon one another, so as to allow the animal, in many cases, to roll itself up after the manner of a wood-louse or hedgehog. The thorax is usually strongly trilobed, and each thorax-ring shows the same triloba-tion, being composed of a central, more or less strongly convex, portion, called the "axis," and of two flatter side-lobes, called the "pleurae."
The " pygidium," or "tail," is usually trilobed also, and, like the thorax, consists of a median axis and of a marginal limb, the composition of the whole out of anchylosed segments being shown by the existence of axial and pleural grooves.