146 



IXVEETEBEATE ANIMALS. 



Order Trilobita. 



The Trilobites constitute a 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. 95). The entire shell or crust is composed of an anterior semi- 

 circular shield, covering the head, a series 

 of movable rings constituting the thorax, 

 and a tailpiece composed of amalgamated 

 segments, and representing the abdomen. 

 On the under surface of the shell nothing 

 had until lately ever been discovered ex- 

 cept the upper lip, but recently limbs and 

 branchiie have been made out. The cephalic 

 shield usually bears a pair of compound 

 eyes, but these are sometimes wanting. It 

 is known that most of the Trilobites pos- 

 sessed the power of rolling themselves up 

 into a ball, much as our modern wood-lice. 

 The Trilobites are only known as occurring 

 in the older rocks of the earth's crust, and they are chiefly character- 

 istic of the period knovn to geologists as the " Silurian." 



Fig. !)5. — Oknus micruriiS. 

 (After S.^lter.) 



Order PHrLLOPODA. 



This is a small order comprising a number of Crustaceans, which 

 are principally found in inland waters, whether fresh or salt, only 

 one form inhabiting the sea. These Crustaceans (fig. 96) have 



'^. ml.— Pliyllopoda. Fairy Shrimp (ChirocepJmlus, or BrailcJiipus dlaphanus). 

 (After Bairil.) 



numerous feet, never less than sixteen in number, and the feet are 

 mostly expanded and leaf-like, and serve as gills or breathing-organs. 

 The body may lie naked, or the head and thorax may be covered by 



