66 INVERTEBRATE ANIMALS. 



II. Suborder Eurypterida. supuf, broad ; nrspov, 

 a wing. The species are known as Palaeozoic fossils. 



IT Four species of Limulus, and a small example of 

 Pterygotus, a genus of fossil Crustaceans, some 

 species of which attain a length of six feet. 



Sub-class MALACOSTRACA. The third and 

 highest Sub-class of the Crustacea. The body-segments 

 (somites) are regarded as being twenty-one in number, 

 of which seven are cephalic (belonging to the head), 

 seven thoracic, and seven abdominal ; but the whole of 

 these can be distinguished in no single example. The 

 modifications of the segments and their appendages are 

 extremely complicated ; one rule, however, obtains in all 

 instances, namely, that each pair of appendages indicates 

 a separate segment. 

 Division EDEIOPHTHALMATA. 'e^pu, a seat ; 



6(p$aXiJi,os, an eye. Eyes not on foot-stalks ; body 



not protected by a carapace. 

 Group 164. — Order ISOPODA. JVof, equal; novg, a foot. 

 The animals of this group generally possess seven more 

 or less uniform pairs of feet, which in the three sections 

 of the order are modified to suit various modes of life. 



I. Cursorial. The Sea-slater, Ligia, common on 

 rocks and walls near the sea. The wood-piercing Isopod, 

 Limnoria terebrans, wherever it prevails, notwithstand- 

 ing its small size, is a most destructive enemy to 

 ships, piers, and timber embankments. The Wood-lice, 

 Oniscus, and the Pill-balls, Armadillo, though terrestrial, 

 breathe by modified gills. 



II. Natatorial. The Sea Pill-balls, Sph(^roma, 

 swim freely, but the Fish-lice, Cymothoa, remain for the 

 greater part of their existence attached to fishes. 



