238- 



ZOOLOGY 



of similar appearance, but with non-glandular epithelium. 

 In some forms with closed air-bladder the anterior end of 

 the organ is forked, and each branch (Fig. 894, a) fits closely 

 against a membranous space in the posterior wall of the 

 auditory capsule, while laterally it extends outwards in the 

 region of the shoulder-girdle, and comes to lie immediately 

 beneath the skin ; in this way varying pressures on the surface 

 of the body are transmitted through the air in the bladder to 

 the auditory organ. In the Carps and Slluroids a 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 

 (Physostomi), and similar vestiges occur in 

 several other genera of the same sub-order, 

 but in all the rest of the order it is entirely 

 unrepresented. On the other hand, Tele- 

 ostei always have a large bulbus aortas, 

 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, except in Amia, is completely non- 

 nervous ; its floor consists of a pair of 

 massive corpora striata (Fig. 895, prs. and 

 Fig. 866, BG.). In most instances the 

 olfactory bulbs are in close apposition with 

 the olfactory region of the prosencephalon 

 without the intervention of olfactory 

 stalks or 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 



-prs 



ipt.l 



cbl 



Fio. 895.— Brain of IiCpi- 

 dosteus, dorsal view, 

 cbl, cerebellum ; c. h. olfac- 

 tory part of prosen- 

 cephalon; di. diencephalon ; 

 m. 0. medulla oblongata ; 

 olf. t. olfactory bulbs ; opt. 

 I. optic lobes ; prs. corpora 

 striata. (After Balfour and 

 Parker, ) 



