METAZOA 141 



the removal of solids — both of solutes which have entered from with- 

 out and of the nitrogenous products of metabolism — is also essential, 

 and in most animals advantage is taken of the outgoing water to remove 

 the solids. It is to provide for this as well as to meet the loss due to 

 evaporation, that terrestrial animals must take in water by the mouth. 



In coelenterates excretion probably takes place from the general 

 surface of the ectoderm, and perhaps also from the endoderm. In 

 triploblastic animals without a perivisceral cavity ectodermal in- 

 growths — the nephridia, mentioned above — permeate the mesen- 

 chyme and perform excretion. In the Nematoda there are lateral 

 ducts in the ectoderm, which probably subserve excretion. In 

 animals with a coelomic perivisceral cavity excreta are shed into the 

 cavity (or carried into it by such cells as the ''yellow cells" of the 

 earthworm) ; and removed to the exterior by nephridia (which may, 

 as in the earthworm, open to the coelom), by the mesodermal coelomo- 

 ducts, or in other ways, as in echinoderms, which shed excreta from 

 the coelomic fluid through the respiratory organs (gills, respiratory 

 trees). In echinoderms also solid excreta, perhaps not nitrogenous, 

 are removed by amoebocytes which pass to the exterior through the 

 gills. In the Vertebrata the excretory portions of the coelom are in the 

 adult separated, and imbedded as the Malpighian capsules in the mass 

 of coelomoducts which forms the kidney. During its passage along 

 the nephridial tube or coelomoduct the fluid containing excreta re- 

 ceives additional substances secreted by the walls of the tube ; and 

 in the terrestrial vertebrates, in which it originates as an exudation 

 filtered out under blood-pressure in the Malpighian capsules, water 

 and some of its solid contents are regained by absorption from it. In 

 the Arthropoda, where the perivisceral cavity is haemocoelic, the 

 excretory organs are still often coelomoducts (segmental organs of 

 Peripatus, antennal and maxillary glands of crustaceans, coxal glands 

 of arachnids) : attached to or imbedded in these are vestiges of the 

 coelom (end-sac). Instead of, or in addition to, these organs, tubular 

 diverticula of the ectodermal or endodermal parts of the alimentary 

 canal often perform excretion in this phylum (Malpighian tubes, 

 certain of the ''hepatic" coeca of crustaceans). In the insects the 

 "fatty body" contains a temporary or permanent deposit of excreta 

 removed from the circulation. In ascidians excreta are similarly laid 

 up as concretions by mesodermal cells. Various other organs which 

 are known or supposed to have an excretory function will be men- 

 tioned in later chapters. The nitrogenous excreta vary in chemical 

 constitution in diff^erent animals. Their variety appears to depend 

 partly on the fact that the products of the decomposition of protein, 

 ammonia compounds, are toxic and accordingly, unless they can be 

 speedily discharged from the body, are converted into such substances 



