CRUSTACEA. 135 



ORDER TRILOBITA. The Trilobites constitute another 

 wholly extinct order of the Crustacea, and deserve a short 

 notice from their great geological importance. They derive 

 their name from the fact that the body exhibits a more or less 

 conspicuous division into a central and two lateral lobes (Fig. 

 55, 1). The entire shell or crust is composed of an anterior 



FIG. 55. Trilobita. 1. Angelina Sedgwickii; 2. Diagram of the cephalic shield of a 

 Trilobite (after Salter). 



semicircular shield, covering the head (Fig. 55, 2), a series of 

 movable rings, constituting the thorax, and a tail-piece com- 

 posed of amalgamated segments, and representing the abdo- 

 men. On the under surface of the shell nothing had ever 

 been discovered except the upper lip, but recently traces of 

 limbs have been made out. The cephalic shield usually 

 bears a pair of compound eyes (Fig. 55, 2 o), but these are 

 sometimes wanting. It is probable that most of the Trilo- 

 bites possessed the power of rolling themselves up into a ball, 

 much as our modern wood-lice. The Trilobrtes are only known 

 as occurring in the older rocks of the earth's crust, and they 

 are chiefly characteristic of the period known to geologists as 

 the " Silurian." 



ORDERS CLADOCERA, COPEPODA, AND OSTRACODA. These 

 orders deserve mention more from the extreme abundance of 

 their commoner forms than for any other reason. They in- 

 clude a number of minute Crustaceans, most of which are 

 commonly called " water-fleas," and abound in fresh waters in 

 most parts of the world. They are, however so small that, 

 though visible to the naked eye, they can only be satisfac- 

 torily examined under the microscope. As an example of the 



