UROGENITAL ORGANS 357 



which the abdominal pores empty, and which, in turn, opens at the tip of a 

 urogenital papilla just behind the anus. 



In the myxinoids the nephridial tubules develop as a continuous series, the 

 organ in the earliest stage known extending over somites 11-80. Later the 

 organ becomes divided into two parts by the degeneration of the intermediate 

 tubules. The anterior part projects into the body cavity and is provided with 

 nephrostomes, while the posterior part, reaching through some twenty or thirty 

 somites, has its tubules strictly segmental, each with a Malphigian body. 

 This is the functional excretory organ. 



The gonads, which are usually unpaired (p. 347) are band-like structures, 

 supported by a fold of the peritoneal membrane (mesorchium or mesoarium, 

 p. 347). The eggs and sperm, when ripe, fall into the coelom and are carried 

 to the exterior by the urogenital pore. The myxinoids have hermaphroditic 

 gonads, the anterior part being ovarian, the posterior testicular. One sex, how- 

 ever, predominates in each individual, except a few which are completely sterile. 

 One view (Nansen) is that the sexes alternate in function (proterandric her- 

 maphroditism). Another is that (Schreiner) there is a rudimentary hermaphro- 

 ditism, the gonad of one sex or the other being degenerate. The eggs of the 

 lampreys are small, those of myxinoids much larger, the latter being enclosed 

 in a horny shell with anchoring hooks at either end. The eggs of the lamprey 

 closely resemble those of the amphibia in their early development. 



ELASMOBRANCHS. — The pronephros is never functional as an excretory 

 organ, but its nephrostomes fuse and form the ostium tubae in the female, by 

 which the eggs are carried from the ovary to the exterior. In these animals 

 there is the typical splitting of the pronephric duct into WolflRan and Mlillerian 

 ducts (p. 342). The functional excretory organs are the mesonephroi and the 

 urinary ducts from the functional part of these may separate from the rest 

 (fig. 378, u) which degenerate with the degeneration or modification of the 

 anterior end of the Wolffian body. The general form of the mesonephroi is 

 modified by the other viscera, and these organs are frequently asymmetrical. 

 Usually the nephrostomes are closed in the adult, but in some {e.g., Acanthias) 

 they persist. The anterior end of each mesonephros is always narrowed, and 

 in the male it and the anterior end of the Wolffian duct form the connexion with 

 the testis, the coiled anterior end of the duct forming the epididymis (fig. 

 378, e) while the mesonephric tissue is frequently called Leydig's gland (fig. 

 379). The urinary ducts fuse behind into a urinary bladder. In the female 

 the bladder opens at the tip of a urinary papilla, but in the male it connects with 

 a urogenital sinus, into which a pair of sperm reservoirs empty, and from this 

 a duct empties at the tip of a urogenital papilla (fig. 378, B). 



As in all other vertebrates the gonads are at first paired and symmetrical, 

 but in some {Trygon, Myliobatis and their relatives) only the left ovary comes to 

 full development, possibly the result of pressure of the spiral valve on the right 

 gonad. With these exceptions the gonads are paired and lie far forward in the 

 body cavity as lobes connected with the body wall by mesorchia or mesovaria, 

 the testes being more variable in position than are the ovaries. 



The MUllerian ducts serve as oviducts, those of the two sides connecting 

 with the coelom by a common opening (fig. 378, 0) the ostiiun tubae abdominale, 



