64 INVERTEBRATE ANIMALS. 



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 EDRIOPHTHALMATA. %«, a seat ; 

 oipdaKfjios, an eye. Eyes not on foot-stalks ; body 

 not protected by a carapace. 



Group 164. — Order ISOPODA. iVoj, equal; ttouj, 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 

 walls near the sea. The wood-piercing Isopod, Limno- 

 ria terebrans, wherever it prevails, , notwithstanding 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, Spharoma, 

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

 greater part of their existence attached to fishes. 



III. Sedentary. The Prawn parasite, Bopyrus, may 

 sometimes be detected under the carapace of a prawn, 

 where the Isopod probably feeds together with, rather 

 than upon, its host. 



Estimated number of species : recent, 210 ; British, 42. 



