ARTHROPOD A 255 



few, but they have been little studied in this country. 

 Any one who will investigate them patiently is sure to 

 make discoveries. Although the group is essentially a 

 fresh-water one, there are very few marine representa- 

 tives, while some are terrestrial, living in damp places. 

 Regarding the arthropods broadly, we divide them into 

 two great series: one adapted to aquatic life and usually 

 breathing by means of gills; the other characteristically 

 terrestrial. The first includes the Crustacea, the sec- 

 ond the Arachnida, Prototracheata or Onychophora, 

 Myriapoda, and insects. On investigation, we find land 

 Crustacea and aquatic arachnids and insects ; so the 

 distinction of habitat is only broadly valid. We find, 

 however, that such animals as the insects and spiders 

 have developed special organs for breathing air, and Develop- 

 certain of them, when living in the water, carry a bubble breathing 

 of air entangled in the hairs of the abdomen. We are structures 

 reminded of the whale, a mammal which has become 

 modified for aquatic existence, but is still obliged to 

 breathe air. Yet many insect larvae, such as those of 

 the may-flies, breathe the oxygen dissolved in the water. 

 These have developed gills, but apparently not very 

 efficient structures, since in many cases the animals can 

 live only in running water, where a new supply of oxygen 

 is continually brought to them. Everywhere the tend- 

 ency is for each group to develop members fitted for 

 every available mode of life, aquatic and terrestrial, 

 free and parasitic, motile and sedentary, limited in this 

 by the ability to vary and by the competition of those 

 which got there first. In the sea the Crustacea are Habitats of 

 dominant, and insects, so successful on the land, are crustaceans 

 practically absent. In the fresh water insects abound, 

 but the Crustacea also are numerous. On the land in- 

 sects may be said to rule, the combined terrestrial 



