THE PHYLA ARTHROPODA AND ONYCHOPHORA 



and the consequent partition of functions, must have occurred reduction in the 

 numl)er of somites; this would have been of survival value in that it led to the 

 formation of a more compact and efficient body. It apparently involved 

 coalescence of segments and their ganglia, as well as eventual elimination 

 of anamorphic growth with its constant addition of posterior segments. 



Similarly widespread changes occurred internally. Coelomic cavities 

 suffered reduction and were replaced by the large blood spaces of the 

 hemocoel. Segmental excretorv organs associated with coelomic pouches 

 became more and more restricted. In the crustacean stem these are now 

 represented by a short and variable series of glandular structures, such as the 

 green glands of the Malacostraca and the maxillary glands of more primitive 

 forms. In chelicerates the coxal glands form a similar series of excretory 

 organs, believed to represent modified nephridia. 



All the foregoing changes were characteristic of aquatic arthropods and set 

 the stage for the rise of the first terrestrial arthropods. In this important 

 epoch, possession of an exoskeleton generally impervious to the passage of 

 water, and of walking legs capable of supporting the weight of the body, took 

 on a new significance. Through many thousands of years, random mutations 

 and the selective action of the terrestrial environment brought about exten- 

 sive internal modifications, notably in the systems concerned with excretion 

 and respiration. Excretion in fresh-water arthropods involves a considerable 

 expenditure of water, easily replaced from the surrounding medium. In 

 terrestrial arthropods the necessity of water conservation made advantageous 

 the suppression of the nephridial type of excretory organ and the perfection 

 of Malpighian tubules. These organs eliminate excreta in a relatively dry 

 state, and in the hind-gut the wastes are subjected to still further dehydra- 

 tion, the extracted water being returned to the blood. Onychophores, like 

 earthworms, have retained nephridia and are restricted to moist terrestrial 

 environments. Blood gills, the characteristic organs of gas exchange in 

 aquatic forms, are too wasteful of water to be serviceable on land. Except in 

 some terrestrial crustaceans, such as sow bugs, which cannot survive in reallv 

 dry conditions, terrestrial arthropods have developed tracheal respiratory svs- 

 tcms or book lungs. Both of these systems could develop through natural selec- 

 tion, having as their chief selective advantage the fact that they operate with verv 

 little loss of water by evaporation. The spiders and insects that have returned 

 to life in fresh water, either as adults or as immature forms, have not re- 

 gained the blood gills of their remote ancestors. They either carrv with 

 them films or bubbles of atmospheric air, as do many beetles and spiders; 

 breathe at the surface, like mosquito larvae and pupae; or, like the naiads of 

 the Odonata and other orders, develop tracheal gills. 



To all of the adaptively advantageous modifications inherited from earlier 

 terrestrial arthropods, insects have added yet another — the power of flight. 

 Together with their complex, innate behavior patterns, this may be regarded 

 as a major factor in the success of this group, as judged from their vast 

 numbers and infinite diversity. 



485 



