NUTRITION 



inner funnel which opens into the body-cavity, a middle 

 glandular section, and an external bladder that ejects 

 the urine by contraction. The disposition of the renal 

 system in the internally articulated vertebrates is very 

 similar to this ; but now complicated structures begin to 

 appear, a pair of compact kidneys (renes), which are 

 made up of a number of branching nephridia. Three gen- 

 erations of kidneys succeed each other, as phylogenetic 

 stages of evolution — first the primary fore-kidneys 

 (protonephros) , in the middle the secondary primitive 

 kidneys (mesonephros) , and last the tertiary after- 

 kidneys (metanephros). The latter are only reached in 

 the three highest classes of vertebrates, reptiles, birds, 

 and mammals. Mollusks also have a couple of compact 

 kidneys. They are developed from a pair of nephridia, 

 the funnels of which open internally into the heart- 

 pouch (the remainder of the reduced body-cavity); 

 at the back they open outward. The Crustacea also 

 have generally a pair of renal canals. On the other 

 hand, the protracheata (the stem-forms of the air-tube 

 animals) have segmental nephridia, a pair to each joint 

 inherited from their annelid ancestors. The rest of the 

 tracheata, the myriapods, spiders, and insects, have, 

 instead of these, Malpighi tubes, funnel-shaped glands 

 that arise from the entodermal rectum, sometimes one 

 pair or less, sometimes a number in a cluster. 



While most plants are purely plasmodomous, and 

 most animals plasmophagous, there are nevertheless in 

 both organic kingdoms a number of species (especially 

 the lower) whose metabolism has assumed peculiar 

 forms by their relations to other organisms. To this 

 class belong especially the saprosites and parasites. By 

 saprosites are understood those plants and animals 

 which feed entirely or mostly on the corpses of other 

 animals, or the decomposed matter which is unfit for 

 the food of higher animals. Among the unicellular 



233 



