94 How Animals Changed 



and, except in slugs, by a spirally coiled shell, which during quies- 

 cent periods may be closed by an operculum. They are commonly 

 nocturnal or crepuscular and are active when the air is humid. Their 

 eggs are laid in the soil or in rotting logs and are protected by 

 tough coverings. Arthropods are protected from desiccation by exo- 

 skeletons and thus readily take up temporary or permanent resi- 

 dence on land. In crustaceans which become adapted to terrestrial 

 conditions gills dwindle and organs (tracheae, branchial tufts) de- 

 velop which permit respiration in moist internal cavities. Excretion 

 through malpighian tubules replaces that through paired metameric 

 kidneys. Insects are dominant land animals and at present share 

 the land habitats with their chief rivals, the vertebrates. Though 

 they originated on land, many have gone into fresh water, and a few 

 types have even become established in the ocean. Noble (1931) has 

 well described the modifications of the vertebrates for terrestrial 

 life as follows: 



"If the modern fish were to be changed to a tetrapod, a number 

 of important transformations of structure would have to be accom- 

 plished. The gills would have to be lost, and the lungs developed 

 and the nasal passages extended to form internal nares for the in- 

 gress of air when the mouth is closed. The fins and body would 

 have to be modified for land locomotion and the integument 

 changed to resist drying. The latter would mean the development 

 of a cornified epidermal covering and a series of integumentary 

 glands discharging by ducts on to the surface, at least over those 

 parts not provided with an armored skin. Special glands would be 

 required to keep the nasal passage and mouth from drying. The 

 eyes, formerly bathed by the water, would be especially sensitive to 

 the new conditions and must either develop a horny, protective cover 

 as in modern snakes or produce softer eyelids out of dermal folds. 

 In either case a lacrimal gland and drain would be needed for 

 cleansing the eyeball. To keep the nasal passage clean a muscular 

 closing device would be required at the outer end of each nasal 



