Whether regarded as an accessory to hearing, or as a means of regulating the distension of the air-bladder, the physiological value of the connexion must be considerable, and on this point it is at least significant that the Weberian mechanism is characteristic of the dominant families of freshwater Teleosts at the present day.[464] [Illustration: FIG. 223.—Diagram to show the Weberian ossicles and their relations to the ear and the air-bladder. _at_, Atrium, an extension of the sinus impar; _a.v.c_, anterior vertical canal; _b.w_, bony wall of the periotic capsule; _d.e_, the medianly-united endolymphatic ducts; _h.c_, horizontal canal; _in_, intercalarium, a third ossicle imbedded in the ligament (_i.lg_) connecting the scaphium with the tripus; _n_, bony nodules on the sides of the complex vertebral centrum; _p.v.c_, posterior vertical canal; _s_, sacculus; _sc_, scaphium; _s.e_, sinus endolymphaticus; _s.i_, sinus impar; _tr.a_, _tr.c_, the anterior and crescentic parts of the tripus; _ut_, utriculus. The radial lines represent the fibres of the dorsal wall of the air-bladder. (From Bridge and Haddon.)] THE OLFACTORY ORGANS.—These organs are essentially a pair of pit-like inpushings of the skin of the ventral side of the head in front of the mouth, with their lining epidermis differentiated into sensory cells separated by supporting cells, and connected with the olfactory lobes of the brain by olfactory nerves. {391}[Illustration: FIG. 224.—Two stages in the development of the olfactory organ and the pituitary involution in _Petromyzon_. A is the earlier, B a much later stage. _br_, Brain; _in_, infundibulum; _l.lp_, lower lip; _ms_, mesenteron; _n_, notochord; _ol.o_, olfactory organ; _pn_, pineal body; _pt.s_, pituitary sac; _st_, stomodaeum; _u.lp_, upper lip. (From Parker and Haswell, after Dohrn.)]