HABITS. 91 



tinguish the sex, especially as external sexual differences are 

 frequently absent. 



Copulatory organs are only found in male Elasmobranchs, in 

 the form of long grooved cartilaginous appendages (claspers) of 

 the pelvic fins. 



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

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

 bear living offspring, which for the most part undergo their em- 

 bryonic development in a dilated part of the oviduct, which 

 serves as an uterus. Reproduction usually takes place only once 

 in the year, most frequently in spring, more rarely in the summer, 

 and exceptionally, as in many of the Salmonidae, 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 spawn- 

 ing. Some make more extended migrations and pass in large 

 shoals over great distances along the sea-coast (Tunny-fish). 

 Others leave the sea and pass up the mouths of rivers, and over- 

 coming 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 (anadromous, as the 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 fresh waters by millions and pass up stream (kata- 

 dromous). The spawn is as a rule fertilized in the water, and 

 thus artificial fertilization and pisciculture are rendered possible. 

 In the viviparous fish, and in the Rays, Chimaera, and Dogfishes, 

 which lay large eggs enclosed in a horny shell, a true copulation, 

 and an internal fertilization of the egg takes place. It is worthy 

 of note that in a few exceptional cases the male undertakes the 

 charge of the brood (Hippocampus, Coitus, Gasterosteus). 



The embryonic development of fishes is principally distin- 

 guished from that of most higher Vertebrates by the fact that 

 neither amnion nor allantois are developed. Both the small eggs 

 of the Teleosteans, which are provided with a micropyle, and the 

 large eggs of the Elasmobranchs, which are surrounded by a hard 

 horny case, contain a large quantity of food yolk, and undergo a 

 partial segmentation. The eggs of Cyclostomes, Ganoids and 

 Dipnoi, however, undergo a total segmentation. As a rule 



