CRUSTACEA OF THE LAND 205 



however, are precisely like tracheae in structure 

 and function, and only differ from the tracheae 

 of the other groups in the fact that they are 

 confined to the appendages, and do not penetrate 

 into the body. Since the Woodlice are a small and 

 highly specialized branch of the Crustacea, we can 

 hardly suppose that they derive their tracheae from 

 any ancestral type which they had in common with 

 the widely different Arachnids, for example ; and if 

 tracheae have been evolved independently in these 

 two groups, there seems no reason why those of the 

 Insects may not have arisen independently of either. 

 This is only one example out of many which go to 

 show that, in attempting to reconstruct the genealogy, 

 or phylogeny, as it is called, of the animal kingdom, 

 we must constantly admit the possibility of " con- 

 vergent evolution." 



Although Woodlice are very common animals, com- 

 paratively little is known of their habits. They 

 seem to live chiefly on vegetable food, and sometimes 

 damage seedlings and tender plants in gardens and 

 greenhouses, but occasionally they are carnivorous, 

 and even cannibalistic, in their habits. A few species 

 live as " guests " in ants' nests, and one of these, 

 the little blind white Platyarthrus hoffmannseggii, is 

 common in many localities in this country. Why 

 the ants tolerate their presence we do not know, 

 for they do not seem to render any service to their 

 hosts, as do the plant-lice and some other insects 



