. Comparative anatomy. Anatomy, Comparative. 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 m
. Comparative anatomy. Anatomy, Comparative. 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 compari
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