xm 



PHYLUM CHORDATA 



231 



chain of bones connects the air-bladder with the auditory organ, 

 forming the Weberian apparatus, the function of which, as of the 

 simpler arrangement described above, is probably " to bring directly 

 to the consciousness of the Fish the varying tensions of the gaseous 

 contents of the air-bladder, due to the incidence of varying 

 hydrostatic pressures." 



The structure of the heart forms one of the most striking 

 differences between the three Ganoid orders and the Teleostei. 

 In Ganoids there is a muscular conus arteriosus with rows of 

 valves, as in Elasmobranchs ; in Teleostei a vestige of the conus 

 containing two rows of valves has been found in Albula (Physo- 

 stomi), and similar vestiges occur in several 

 other genera of the same sub-order, but in 

 all the rest of the order it is entirely un- 

 represented. On the other hand, Teleostei 

 always have a large bulbus aortse, formed 

 as a dilatation of the base of the ventral 

 aorta. 



In the brain the cerebellum and optic 

 lobes are usually large ; the diencephalon is 

 well developed in Ganoids, almost obsolete 

 in Teleostei. In the Teleostei and Ganoidei 

 the prosencephalon has the general features 

 which have been described in the account of 

 the brain of the Trout : it is not divided 

 into hemispheres and has a roof which, ex- 

 cept in Amia, is completely non-nervous ; 

 its floor consists of a pair of massive corpora 

 striata (Fig. 906, prs., and Kg. 877, BG.). 

 In most instances the olfactory bulbs are in 

 close apposition with the olfactory region of 

 the prosencephalon without the intervention 

 of olfactorv stalks or tracts; but in some i- optic .lobes; prs. corpora 



. J , i n i /TV f\f\K if \ j.t- striata. (After Balfour and 



cases, as in the Cod (Fig. 905, elf. p.), they p ar ker.) 

 are borne on long olfactory peduncles or 



olfactory tracts. The Ganoids agree with Elasmobranchs in the 

 fact that the optic nerves form a chiasma, while in Teleostei 

 they simply cross one another or decussate. Here also, however, 

 the distinction is not quite absolute, since in the Herring and 

 some other Physostomes one nerve passes through a slit in the 

 other. In some Plectognaths the spinal cord undergoes a remark- 

 able shortening : in a Sun-fish 2| metres in length and weighing 

 a ton and a half the cord is only 15 millimetres long, being 

 actually shorter than the brain. 



Urinogenital Organs. The kidney (Fig. 876, led) is formed 

 from the mesonephros of the embryo, and usually attains a great 

 size ; the pronephros usually atrophies. The ureter (ur.) is the 



p 2 



[ft. 906. Brain of 

 dosteus, dorsal view. 

 cbl. cerebellum ; c. h. olfac- 

 tory part of prosen- 

 cephalon ; di. diencephalon ; 

 m. o. medulla oblongata ; 

 oJf. I. olfactory bulbs ; opt. 



