292 



CHORDATE ANATOMY 



protonephridia resemble the branched tubules of flatworms not only in 

 structure but also in function. (Fig. 263) 



Metanephridia. Many annehds 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 

 oUgochaetes 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. CiHa surrounding 

 the nephrostome sweep wastes from the body-cavity into the nephridium 



Fig. 262. — A diagrammatic cross section of an annelid, showing the coclomoducts. 

 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. Probably 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 reproductive 

 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. Consequently, 

 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 



