2IO 



ANIMALS 



quiescent, may have its demands for oxygen satisfied by ab 

 sorption. through the skin. But the more compactly buil 

 animals, even when not large, are provided with special organ: 

 for the absorption of oxygen. The earthworm is among th< 

 largest of animals destitute of such organs. But the earth worn 

 is unable to absorb enough oxygen when in water and wil 

 ultimately drown. Nereis possesses a pair of flat plates ii 



FIG. 116. 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; m, m', muscles; n.c., nerve cord; p.s., pericardial sinus 

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

 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 ir 

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

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

 movement by which the water is kept in motion. 



