152 THE STUDY OF ANIMAL LIFE CHAP. 



other circulating fluid, took on the function of distributing 

 the oxygen to the tissues and collecting the carbonic 

 acid ; and special surfaces were developed where the 

 blood was spread out and diffusion facilitated. These 

 surfaces formed all sorts of gills in aquatic animals, and 

 all sorts of lungs in terrestrial animals. The simplest 

 of all arrangements we see in animals like the earth- 

 worm where the respiration is cutaneous, the blood 

 circulating close to the surface all over the skin. To 

 this primitive method there is often a relapse at much 

 higher levels ; thus the newly hatched tadpole and the 

 adult frog lying passive in the mud in the winter-time 

 are alike in showing cutaneous respiration. Of great 

 importance is the presence of special respiratory pig- 

 ments which have a strong affinity for oxygen and are 

 therefore able to capture it readily, passing it on by and 

 bye to the tissues. These pigments are usually in cells 

 of the blood (as in red blood-corpuscles) or of the body- 

 cavity fluid (as in sea-urchins), or in the fluid of the 

 blood (as in earthworms) ; but they may occur in animals 

 without blood or circulating fluid, as in sponges. Haemo- 

 globin, the red blood-pigment characteristic of Verte- 

 brates, makes its first appearance in Nemertean worms ; 

 an analogous bluish pigment, called ha?mocyanin, is 

 common among Invertebrates. It is interesting to notice 

 that the respiration of numerous simple animals, e.g. 

 Stentor among Protozoa, Hydra among Zoophytes, and 

 Convoluta among Plaiiarians, is in great part secured by 

 partner-Algae with chlorophyll, which evolve oxygen in 

 the sunlight and use the CO 2 produced by their partners. 

 7. Excretion. Life is activity ; this involves expendi- 

 ture of energy ; the supply of this is obtained from the 

 reactions of chemical substances ; and this implies the 

 formation of waste-products. One of these is carbon- 

 dioxide already referred to, but there are also nitrogenous 

 waste-products which must be got rid of or converted 

 into something not obnoxious. In plants they are 

 deposited internally as crystals or in some other form ; 

 in some animals like Ascidians there seems to be a reten- 

 tion of nitrogenous waste in small reservoirs from which 



