148 PISCES. 



cavity of the sac which becomes greatly swollen at the breeding 

 time. The testes on the other hand are, except in the Cyclostomes, 

 paired, and they are composed of transverse canals or vesicular 

 cavities. 



In the simplest case the testes and ovaries have no special ducts, 

 but the genital products are dehisced from the wall of the gland 

 into the body cavity, whence they pass out to the exterior through a 

 genital- pore situated behind the anus (in I'yclostomes, Eels, and 

 female Salmon). As a rule, however, generative ducts are present ; 

 they may either be direct prolongations of the genital glands as in the 

 Teleosteans. or as in the Ganoids, female Plagiostomes and Dipnoi 

 independent canals which begin with a free funnel-shaped opening 

 into the body cavity (Miillerian ducts). In the Teleosteans the two 

 oviducts as well as the vasa deferentia unite to form an unpaired 

 duct which opens to the exterior on the urogenital papilla between the 

 openings of the anus and the urinary duct ; in the Ganoids, on the 

 other hand, as well as in the Plagiostomes and the Dipnoi a common 

 cloaca is formed. Accessory external copulatory organs are only 

 found in the male Plagiostomes, in the form of long grooved 

 cartilaginous appendages of the ventral fins. 



Most fishes are oviparous ; only a few Teleosteans, as AnaUeps, 

 Zoarces, the Cyprinodonta, etc., and a great number of the Sharks, 

 bear living offspring, which for the most part undergo their embiyonic 

 development in a dilated part of the oviduct which serves as a uterus. 

 Reproduction usually takes place only once in the year, most fre- 

 quently in spring, more rarely in the summer, and exceptionally, as 

 in many of the Salmonidce, in winter. Many fishes, especially the 

 males, undergo changes of colour and develop growths of skin at the 

 spawning time. The two sexes often assemble in great shoals and 

 seek out shallow places near the banks of rivers or near the sea coast 

 (Herrings) for spawning. Some make more extended migrations 

 and pass in great shoals over great distances along the sea coast 

 (Tunny-Fish). Others leave the sea and pass up the mouths of 

 rivers, and overcoming great obstacles (Salmon leaps) make their way 

 up into the smaller streams in which they deposit their spawn in 

 sheltered places where the food is plentiful (Salmon, Sturgeon, etc.). 

 The Eels on the other hand migrate from the rivers into the sea, and 

 in the following spring the young Eels enter the freshwater by 

 millions and pass up the stream. The spawn is as a rule fertilized in 

 the water, and thus artificial fertilization and pisciculture is rendered 

 possible. In the viviparous fish, and in the Rays, Chimcera, and 



