GANOIDEI. 



165 



gills, on the other hand, lie, as in the Teleosteans, freely in a 

 branchial cavity beneath a branchial operculum, to which a large 

 gill containing venous blood is often attached. This respiratory 

 accessory gill (opercular gill) is wanting in Amia and Spatularia, 

 and must be distinguished from the pseudobranch of the spiracle, 

 which may be present together with it. 



All the Ganoids possess a swimming bladder with a ductus pneu- 

 maticus and two peritoneal canals (abdominal pores], which open at 

 the sides of the anus (as in Chimcera and Plagiostomi). The optic 

 nerves do not simply cross over one another, but form a chiasma 

 with partial exchange of the fibres. The generative organs present 

 many noteworthy peculiarities. There are two ovaries and the ripe 

 eggs escape into the abdominal cavity. Thence they pass into an 

 oviduct [Miillerian duct] which begins with a funnel-shaped opening 

 into the body cavity and opens behind into the urinary duct or into 

 the corresponding cornu of the urinary bladder (Spatularia, Lepi- 



FIG. 6)3. Acipenser ruthenus (after Heckel and Kner). 



dosteus), or unites with the oviduct of the opposite side and opens 

 behind the anus by a single genital pore into which the short urethra 

 also opens. (Hyrtl.) In the two first cases a urogenital canal leads 

 from the bladder to a urogenital pore placed behind the anus. In the 

 male it is remarkable that the same abdominal funnels [Miillerian 

 ducts] also function as seminal ducts. [It has been shown by Balfour 

 and Parker (Structure and Development of Lepidosteus, Phil. Trans., 

 1882), that in Lepidosteus at any rate the testis is connected with 

 the Wolffian body by a testicular network.] 



Tribe 1. Chondrostei. Cartilaginous Ganoids with persistent 

 notochord. Branchiostegal rays scanty or absent. Caudal fin 

 heterocercal, with fulcra. Cranium cartilaginous, covered by dermal 

 bones. The teeth are small or altogether absent. The skin is 

 naked or has osseous plates instead of scales. 



. Fam. Acipenseridae (Sturgeons). Acipenser sturio L., Sturgeon; A. rutJienus 

 L., Sterlet (fig. 608) ; A. huso L. (Hausen), Scaphirliynclms cataphractut 

 Gray, Mississippi. 



