204 THE LIFE OF CRUSTACEA 



body, and by lacking the plate-like pleopods. The 

 resemblance between the two animals can hardly be 

 regarded as a case of " mimicry," since there is no 

 reason to believe that either benefits by its likeness 

 to the other. As in so many other cases of " con- 

 vergent resemblance " between animals of different 

 structure, it does not seem possible to get beyond 

 the vague suggestion that a similarity in habits may 

 have led, in some way that we do not understand, to 

 a similarity in appearance. 



The presence of air-tubes in the pleopods of many 

 Woodlice raises some questions which are of impor- 

 tance with reference to the classification of the 

 Arthropoda as a whole. The Six-legged Insects, 

 most Spiders and many of their allies, the Centi- 

 pedes and Millipedes, and the worm-like Peripatus, 

 all breathe air by means of fine tubes which pene- 

 trate throughout the body, and bring the air into 

 close contact with the tissues. These tubes, which 

 are known as "tracheae," arise as ingrowths of the 

 outer layer of the embryo, and are lined by a delicate 

 continuation of the external cuticle. It has been 

 held by some zoologists that so peculiar a system of 

 breathing organs must indicate a common descent 

 of the animals that possess them, and accordingly 

 it has been proposed to separate the Insects, Arach- 

 nids, Myriopods, and Peripatus, as a group, Tracheata, 

 from the Crustacea and some other Arthropods which 

 have no tracheae. The air-tubes of the Woodlice, 



