Ichthyopsida 433 



The air-bladder of teleosts is always dorsal to the digestive tube. 

 In sonic groups its embryonic communication with the lumen of the 

 digestive tube is permanently retained ; in others it becomes secondarily 

 closed, in form, and in details of structure and relations, it exhibits 

 great diversity among the many groups of Teleostei. Especially sig- 

 nificant are the relations of the air-bladder to the ears. Many teleosts 

 have anatomic arrangements which apparently utilize the static sen- 

 sory mechanism of the ear (see p. 207) for control of gas pressure in the 

 air-bladder. In some cases a pair of branches or diverticula project 

 forward from the air-bladder, and the anterior end of each diverticulum 

 abuts against the perilymph space of the otic sac of that side of the 

 head. There is no open communication — the anterior end of the diver- 

 ticulum is a " blind " end. In catfishes (Siluridae) and the carplike 

 fishes (Cyprinidae), a skeletal mechanism intervenes between air- 

 bladder and otic sacs. Just beneath the vertebral column are two chains 

 of small bones articulated together — the "Weberian ossicles" (Fig. 

 336). The relatively large posterior ossicle of each chain is attached to 

 the wall of the air-bladder. The very small but elongated anterior 

 ossicle penetrates into the otic region and terminates against a mem- 

 brane, on the other side of which is perilymph. (Incidentally, the right 

 and left saccular regions of the ears are transversely joined beneath the 

 brain so that the right and left otic cavities are continuous.) A third 

 small ossicle is intercalated between the posterior and anterior mem- 

 bers of each chain. 



The structure of the Weberian apparatus indicates that effects 

 of changes of pressure in the air-bladder are transmitted by the ossicles 

 to the fluids (perilymph and endolymph) of the otic sacs. By this 

 means, the reflex static mechanism of the ears is activated and appro- 

 priate adjustments of pressure in the air-bladder are automatically 

 made. The origin of the two series of Weberian ossicles is obscure. 

 They seem to derive their skeletal material from the closely adjacent 

 region of the embryonic vertebral column or possibly, in part, from 

 the dorsal ends of ribs. 



In older classifications it was a common practice to subdivide 

 Teleostei into two groups distinguished by the relation of the air- 

 bladder to the digestive tube in the adult. The Physostomi were 

 teleosts whose air-bladder remains in open communication with the 

 digestive tube, as in the catfishes (Siluridae), pickerel, carp, salmon, 

 trout, herring, and others. In the Physoclisti the air-bladder loses its 

 embryonic connection with the digestive tube and becomes a closed 

 sac. The great majority of teleosts are physoclistic. It has come to be 

 realized, however, that, for purposes of genetic classification, these rela- 

 tions of the air-bladder to the digestive tube are not dependable. In 



