THE UROGENITAL SYSTEM 



401 



protonephridia resemble the branched tubules of flatworms not only in 

 structure but also in function. (Fig, 331) 



Metanephridia. Many annelids also have metanephridia, which are 

 nephridia devoid of solenocytes and opening not only to the exterior 

 but also into the coelom. The internal aperture is the nephrostome. In 

 oligochaetes the metanephridia are much convoluted, and the nephro- 

 stome opens into the coelom of the segment anterior to the one which 

 contains the external aperture. The metanephridia are paired and occur 

 in most segments of the body throughout its length. Cilia surrounding 

 the nephrostome sweep wastes from the body cavity into the nephridium 



lORSAL BLOOD VESSEL 



'CIRCULAR MUSCLES 



Fig. 330. — A diagrammatic cross section of an annelid, showing the coelomoducts. 

 With a ciliated opening into the coelom and an external aperture, each coelomoduct 

 serves as an outlet of the reproductive cells — eggs or spermatozoa. They are homolo- 

 gized with the renal tubules of chordates. (Redrawn after Lang.) 



and thus to the exterior. Like the protonephridia, the metanephridia are 

 ectodermal in origin. It is assumed that they are genetically related to 

 the protonephridia. 



Coelomoducts. In addition to the two kinds of nephridia just described, 

 many invertebrates have a third type of tubules, coelomoducts. These in 

 annelids, however, are not excretory in function but serve as repro- 

 ductive ducts. Like the metanephridia, they have coelomostomes 

 which are ciliated and open into the coelom. Like nephridia, also, they 

 are paired and metameric in arrangement. Their mesodermal origin, 

 however, makes it impossible to compare them with nephridia. Conse- 

 quently, they must be considered as novelties first appearing in annelids. 

 Their special interest to morphologists lies in the fact that they resemble 

 the kidney tubules of vertebrates and are consequently regarded as proto- 

 types of the latter. If this comparison is valid, we must assume that, in 

 the course of phylogenesis, they have changed from a reproductive to an 



