2IO 



ANIMALS 



quiescent, may have its demands for oxygen satisfied by ab- 

 sorption through the skin. But the more compactly built 

 animals, even when not large, are provided with special organs 

 for the absorption of oxygen. The earthworm is among the 

 largest of animals destitute of such organs. But the earthworm 

 is unable to absorb enough oxygen when in water and will 

 ultimately drown. Nereis possesses a pair of flat plates in 



i 



Fig. ii6. — Cross section of crawfish in the thoracic region, a, Appendage; 

 c, carapace; cf, part of carapace covering the' gill chamber; d, digestive tract; 

 g, gill; h, heart; /, liver; ?n, m' , muscles; w.c, nerve cord; p.s., pericardial sinus; 

 r, gonad; st, sternal artery; va, ventral artery; vs, ventral blood sinus. (From 

 Galloway, after Lang.) 



each segment, one on each parapodium, which are richly sup- 

 plied with capillaries lying very near the surface. These supple- 

 ment the general body surface in the absorption of oxygen. 

 Even here the worm feels the necessity of keeping the water in 

 motion in order to bring in fresh supplies of oxygen. When 

 the animal is at rest the body keeps up a rhythmical undulating 

 movement by which the water is kept in motion. 



