222 



ZOOLOGY 



SECT . 



jors 



organ. In the Carps and Siluroids a chain of bones connects the 

 air-bladder with the auditory organ, forming the Weberian 

 iippamtm, 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, one of the Herring 

 family, but in no other member of the order. On the other hand, 

 Teleostei always have a large bulbus aorta?, formed as a dilatation 

 of the base of the ventral aorta. 



In the brain the cerebellum and optic lobes are large ; the 

 diencephalon is well developed in Ganoids, almost obsolete in Tele- 

 ostei. In Ganoids there is an unpaired 

 prosencephalon, which may be produced 

 into lobes (Fig. 845, prs.} and has a nori- 

 nervous roof, giving off anteriorly a pair 

 of cerebral hemispheres (c.h.) into which 

 the prosoccele is continued as a pair of 

 lateral ventricles or paracoeles ; thus the 

 fore-brain of Ganoids presents many re- 

 semblances to that of the Lamprey. In 

 Teleostei (Fig. 818) there are no cerebral 

 hemispheres, but only an undivided pro- 

 sencephalon with a non-nervous roof or 

 pallium, and with its floor raised into large 

 rounded corpora striata. 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 dis- 

 tinction is not quite absolute, since in the 

 Herring and some other Physostomes one 

 nerve passes through a slit in the other. 

 In some cases the olfactory lobes spring 

 directly from the prosencephalon, as in 

 the Trout ; in others they are borne on long 

 olfactory peduncles (Fig. 844, olf. p.), as in 



the Cod. In some Plectognaths the spinal cord undergoes a re- 

 markable shortening : in a Sun-fish 2 J metres in length and 

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

 being actually shorter than the .brain. 



Urino-genital Organs. The kidney (Fig. 817, kd.) is formed 

 from the mesonephros of the embryo and usually attains a great 



Tl 



FIG. 845. Brain of Lepi 

 dosteus, dorsal view. 

 cbl. cerebellum ; c. li. cere- 

 bral hemispheres"; di. dien- 

 cephalon ; m. o. medulla 

 oblongata ; olf. I. olfactory 

 lobes ; opt. I. optic lobes ; 

 prs. lobes of prosence- 

 alfour 

 ker.) 



phalon. (After Balfour and 

 Part 



