]26 INTRODUCTION TO CLASSIFICATION. 



Most of these are aquatic and marine animals ; the Wood- 

 lice are their terrestrial representatives. 



7. The MEROSTOMATA. The body is divided into an anterior 

 shield-shaped portion, which bears the sessile eyes on its upper 

 surface ; a middle part is formed by the rest of the somites of 

 the body, and there is a terminal broad, or spine-like, telson, 

 In the living King Crab (Limulus) there are thirteen pair of 

 appendages. The five anterior pair belong to the head, but all 

 have the form of locomotive limbs. The five posterior pair are 

 broad lamellae, united in the middle line, on the posterior faces 

 of which the gills are borne. The heart is elongated and eight- 

 chambered. In the extinct Eurypterida only the head is pro- 

 vided with distinct appendages, and, of these, the posterior pair 

 are styliform, or paddle-like, swimming organs. 



8. The COPEPODA are minute aquatic crustaceans, which 

 inhabit both the sea and fresh-water. Many, like the com- 

 mon Cyclops, are actively locomotive animals, while many, 

 on the other hand, are among the most sluggish and strangely- 

 modified of fixed parasites. These last have been termed 

 Epizoa. 



The locomotive forms have not more than eighteen so- 

 mites in the body, the anterior part of which is covered by a 

 carapace provided with a single, or double, median eye upon its 

 dorsal surface. The antennules are large oar-like appendages, 

 and are the chief agents in locomotion, which is aided by the 

 paddle-like posterior thoracic members. 



The heart is short, and there are no distinct respiratory 

 organs. The sexes are in distinct individuals; the female, 

 among the parasitic forms, being often very much larger than the 

 male. The larvae leave the egg as oval bodies, provided with 

 two or three pair of oar-like limbs. 



9. The TRILOBITA are Crustacea which, like the Eurypterida, 

 have been extinct since the palaeozoic epoch. The form of the 

 head is as in the Merostomata ; in most cases it bears sessile 

 eyes of a similar character. On its under surface a large labrum 

 is attached, but no jaws or other appendages have yet been 

 discovered, and the ventral wall of the body seems to have 

 been completely membranous, The cephalic shield is dis- 



