TELEOSTOMOUS FISHES. 247 



in Polypterus it comes from the radix aortse, and therefore it 

 receives only arterial blood. The bladder serves as a hydro- 

 static apparatus, but there is also evidence to show that at least 

 in some fishes it is to some extent respiratory as well. For the 

 relations of the bladder to the ear, see p. 255. 



The brain is noticeable for the large size of the optic lobes 

 and the cerebellum. The cerebrum is rudimentary, and consists 

 of merely corpora striata and a non-nervous pallium in the tele- 

 osts, but in the ganoids larger hemispheres occur. In the 

 ganoids there is a true optic chiasma, but in the teleosts the 

 optic nerves cross (p. 61). The twixt brain is short. The olfac- 

 tory lobes in most teleosts and in the ganoids are joined to 

 the cerebrum ; but in a few forms a long olfactory tract inter- 

 venes. 



The urogenital organs of the teleostomes will repay further 

 study, for there are many points as yet in doubt. The perma- 

 nent excretory organ is the mesonephros ; only in Fierasfer and 

 Dactyloptertis does the pronephros retain its excretory functions, 

 In all others, while it may be of large size, it degenerates into 

 a lymphatic or adenoid structure. The pronephric ducts never 

 divide into Miillerian and Wolffian ducts, but serve solely as 

 ureters. Usually the two unite behind and form a urinary 

 bladder of some size, the common opening being, except in 

 a few teleosts, behind the vent.' The usually paired gonads 

 vary in the way in which their products reach the exterior. In 

 the female salmonids and eels, the eggs are discharged directly 

 into the ccelom, from which they escape into a urogenital sinus 

 by means of a pair of slit-like openings, often called pori abdom- 

 inales, but which are apparently not homologous with the sim- 

 ilarly named openings in the elasmobranchs. In most ganoids 

 and in a few teleosts, two longitudinal folds arise in the peri- 

 toneum, the edges of which unite so that a tube, the oviduct, 

 results, which opens freely into the ccelom. In most teleosts, 

 however, these folds are continued to the ovary, so that the 

 eggs do not pass into the general body cavity, but fall at once 

 into these tubes, the lumen of which is, as is readily seen, a part 



1 In Pediculati and some symbranchs and plectognaths the urinary opening is in the 

 hinder end of the intestine. 



