AIE-BLADDER IN NOTOPTERUS BORNEENSIS. 529* 



the degree of intimacy and perfection of the physiological 

 connection between the two organs. 



On this point it may be mentioned that in Pseudophycis 

 hacchus, as described by Parker (op. c^Y.), the anterior extremity 

 of each auditory caecum forms a thickened pad which is closely 

 applied to the membranous sheet closing the corresponding 

 auditory fontanelle. From this account it may be legitimately 

 inferred that both the outer fibrous coat of the caecum and 

 the tunica interna are applied to the fontanelle and its mem- 

 brane. In Sparus (Weber, op. cit.), on the contrary, the tunica 

 externa of the auditory caecum ceases at the margins of the 

 fontanelle, and only the extremely thin tunica interna is applied 

 to the membrane which closes the aperture. So far as this point 

 is concerned Notopterns closely resembles Sparus ; but in the 

 Clupeidae, aud possibly in Hyodon, the connection of the two 

 organs becomes even more intimate, inasmuch as it is effected 

 by the actual apposition of the limiting walls of auditory caeca, 

 and utricular outgrowths. 



The derivation of the second and more intimate type of coq- 

 nection between the air-bladder and the auditory organ from the 

 first method is easy to imagine, as the process simply involve& 

 the atrophy of the fibrous membranes closing the auditory 

 fontanelles, so as to admit of the actual contact of the two 

 organs by the direct apposition of outgrowths from each. A 

 much more difficult problem is the genesis of the third method 

 by means of Weberian ossicles, and so far no satisfactory solution 

 has yet been offered. There is every probability, however, that 

 the Weberian mechanism has been independently evolved, but 

 that the initial stages of its evolution have not yet been dis- 

 covered, if they exist, in any Pishes at present living. 



In his account of the connection between the air-bladder 

 and auditory organ in Pseudophycis haccJius, Parker {op. cit.) 

 remarks : " The anterior end of the air-bladder fits closely against 

 the hinder end of the skull aud is produced outwards into paired 

 pouches which are in contact with the thin skin beneath the oper- 

 culum and in front of the shoulder-girdle." To this arrangement 

 Notopterus offers a parallel in the close relation of the auditory 

 caeca to the caecal diverticula of the branchial cavity, and, there- 

 foie, to the external medium in which the fish lives. The 

 Siluroids (3) also afford examples of a somewhat similar 

 modification. In the majority of these Teleosts the lateral walls 



