GENERAL ORGANOLOGY 



105 



important influence on their structure. When neither are developed 



the excretory tubules, in order to remove the excreta from the tissues, 



must branch and penetrate the body in all directions like a drainage system, 



being frequently connected in a network recalling the blood-capillaries 



{protoncpliridia or water-vascular i-\'5/««of parenchymatous worms, fig. 68), 



The canals begin with closed tubes, which are 



provided internally at the end with a bundle of 



actively vibrating cilia, the 'flame' (fig. 70). 



These flame cells are replaced in many proto- 



nephridia ('head kidneys' of many annelid 



larvEe) by solenocytes (fig. 69), cells with a 



flagellum enclosed in a tube. One or more 



main trunks lead from the canal system to the 



exterior. A little before the external opening 



(excretory pore) there is frequenUy a contractile 



enlargement, the urinary bladder. 



With the appearance of a ccelom there is a 

 central place for the collection of excreta. 

 The nephridia or segmental organs are usually 

 simple (rarely branched) tubes, open at both 

 ends. One opening is external (fig. 71), the 

 other communicates with the coelom by micans 

 of a ciliated funnel, the nephrostome, a wide 

 mouth with active cilia which connects with 

 the canal of the tube. Through this the excre- 

 tion is carried to the outside. 



The excretory organs {kidneys) of verte- 

 brates are derived from such nephridia. The 

 fact that in the embryos (and frequently in the 

 adults) these open into the ccelom by nephro- 

 stomes makes it probable that also in the 

 vertebrates the coelom was once important in 

 excretion (fig. 72). The increasing import- 

 ance of the blood-vessels which envelop the nephridial canals and bring 

 to them the waste matter taken from the tissues is probably the cause 

 of the loss of connection of the kidneys with the ccelom by degeneration 

 of the nephrostomata. The relation of the blood-vessels to the 

 nephridial tubes becomes specially close by the development of the 

 glomeruli {Malpighian corpuscles), bundles of capillaries carrying the 

 walls of the canal before them and so projecting into the lumen of the 

 tube. Since the nephridial tubules of the vertebrates open into a com- 



/ 



/> ' 



P^IG 6S — Distomum tiep- 

 aticum with water-vascular 

 system (from Hatschek). />, 

 porous e.xcretorius; 0, mouth. 



