xii ANIMAL LIFE 



blood pigments — The respiration of free-living and burrow- 

 ing Crustacea ; of bivalve and univalve molluscs ; of cuttle- 

 fish — Air-breathing worms, crabs, and molluscs — The 

 respiration of fish ; their gills and muscle-pigments — 

 Adaptations to avoid the suffocating influences of drought 

 on fish-life — The air-bladder and its gases ; its varying 

 uses, leading to the formation of lungs — The respiration of 

 amphibia and the conquest of the land — The breathing of 

 reptiles — The evolution of more perfect lungs in birds and 

 mammals associated with the development of the voice anil 

 of a higher organisation — The maintenance of the heat 

 produced by breathing and the production of a constant 

 temperature 104 



CHAPTER VII 



THE SEXSES OF ANIMALS 



The orderliness of animal organisation implies controlled 

 adaptation — The nervous system a visible sign of this 

 harmonic grace — It mediates between the stimuli that fall 

 upon an animal and the resulting responses — It is the 

 seat of traditional and individual memory, and sets goin" 

 periodic as well as immediate actions — The nature of 

 periodic nervous action — Organic memory — The light it 

 throws on animal development — The nervous system the 

 earliest and most important organ to appear — The influence 

 of glands in stimulating the development of the body 



The senses considered as the necessary means of maintaining 

 a station in life, gaming food, and avoiding danger — These 

 senses bound up with essential adjustments to meet the 

 exigencies of life — The contrast between the few essential 

 sensations common to all living things and the vast array of 

 organised form in which they find expression — The habits 

 of a shrimp or prawn taken as an example of the way in 

 which the conduct of these animals is built up out of 

 responses to light, pressure, taste 



The double nature of stimuli that affect animals — Stimuli 

 from without that affect the skin, and others, from within, 

 that affect the internal organs— The correlation between 

 the two effected by the nervous system— The skin as the 

 seat of origin of sense-organs for interpreting external con- 



