THE DIGESTIVE AND RESPIRATORY SYSTEM 311 



sarily the same, yet until the subject has been thoroughly in- 

 vestigated, the latter may be assumed to be the case. 



In several ganoids either one or the other of these pneu- 

 matic systems becomes complex in character and serves as an 

 accessory respiratory organ. The air-bladder functions as a 

 lung; it becomes honeycombed with connective tissue parti- 

 tions, and is profusely vascular, thus forming an organ of far 

 greater functional activity than the definite lungs of many 

 amphibians ; corresponding to this its larynx, the regulator of 

 the air supply, develops an extensive set of muscles and masses 

 of fibro-cartilage. That such a structure, when dorsal in 

 position, as in Lepisosteus, cannot be the precursor of the 

 final pulmonary system of higher forms is self-evident, but 

 when such an organ is ventrally placed, thus corresponding 

 exactly to the embryonic stages of the latter, as in Polypterus, 

 such an homology, although not definitely proven, is very 

 likely. As for the dorsal system, there is no indication that 

 it is represented in any way above the fishes. 



If, however, the ventral air-bladder of Polypterus is identi- 

 cal with the paired lungs of higher forms (which begin as a 

 single median diverticulum that divides later into two 

 branches), the larynx can be the same only in respect to its 

 opening, the glottis, since the accessory parts, that form the 

 functional organ, are derived from two totally different sources 

 in the two cases. In the fish larynx the hard parts are derived 

 from the adjacent connective tissue, and are composed of 

 fibro-cartilage, which represents as it were the first stage in 

 cartilage formation and differs but little from a compact form 

 of simple connective tissue. The muscles are evidently slips 

 differentiated from the muscular walls of the pharynx. That 

 this forms a very effective organ cannot be denied and, had 

 no. better material for a laryngeal mechanism been furnished, 

 that of the ganoid with its fibro-cartilage and slips of pharyn- 

 geal muscle would have undoubtedly developed to fill all the 

 needs of a pulmonary system, even including the functions of \ 

 voice and speech. It chanced, however, that at this period, 

 the fifth branchial arches with their accompanying muscles, 



