442 GENERAL CONSIDERATIONS 



have no specialized cells for elimination, but in flatworms there is a simple 

 type of excretory organ known as a protonephridium, represented by the 

 flame cell of the planarian. Similar structures are present in nemathel- 

 minths, nemertines, and rotifers. In higher invertebrates a structure 

 known as a metanephridium, representing a collection of protonephridia, 

 is the excretory organ. In annelids these structures, which have been 

 described as nephridia, are represented in nearly every metamere. 

 Nephridia are also present in brachiopods, bryozoans, and mollusks, 

 though there is only a limited number of pairs. In echinoderms excre- 

 tory cells, the amebocytes, carry on elimination, while in myriapods, 

 insects, and arachnids malpighian tubules, and in the crustaceans green 

 glands, perform this function. 



Nephridia or excretory cells also carry on elimination in the lower 

 chordates but in vertebrates there is a higher type of structure known as a 

 kidney, or nephros. This contains tubules originating in a certain num- 

 ber of body segments and is called, according to its position in the body, 

 a pronephros, mesonephros, or metanephros (Sec. 349, Fig. 216). The 

 pronephros is functional only in the myxinoids, while a mesonephros 

 is the functional kidney in lampreys and in all forms up to and including 

 amphibians. This is replaced by a metanephros in reptiles, birds, and 

 mammals. The pronephros and mesonephros, however, appear in the 

 embryonic stages of these higher forms and vestiges of them remain 

 even in the adults. 



478. Reproductive System. — In the least differentiated protozoans 

 the individuals are all alike and reproduction is purely asexual. In 

 the higher protozoans reproductive cells, which are essentially sexual 

 organisms, are distinguishable as macrogametes and microgametes, 

 and reproduction becomes, therefore, sexual. In the lower metazoans 

 asexual reproduction still occurs; nevertheless, even in the lowest of the 

 Metazoa reproduction by means of specialized sex cells becomes a 

 prominent method of multiphcation, and in higher forms it becomes 

 the only type. In urochordates, however, asexual reproduction again 

 appears. 



In coelenterates the sex cells are derived from interstitial cells, and the 

 mass of cells in which they are produced is either the testis or the ovary, 

 though here these are not, strictly speaking, organs. In the flatworms 

 not only do the testes and ovaries become organs but a whole system of 

 accessory organs arises and in these animals the reproductive system is the 

 most highly developed of all systems. In higher forms the number and 

 variety of accessory organs vary considerably with the type of habitat 

 and the conditions under which reproduction takes place. 



In many of the lower Metazoa a monecious condition prevails, testes 

 and ovaries being found in the same individual, though as a rule self- 

 fertilization is avoided by structural conditions which prevent it or by 



