286 



FISHES 



In addition to the normal gills there is also a hyoidean pseudo- 

 branch. As in other Dipnoi, an operculum forms the outer 

 wall of the branchial cavity, and leaves but a narrow, slit-like 

 external branchial aperture. 



In Protopterus 1 the number of branchial arches is increased 

 to six, but, in consequence of the closure of the hyobranchial 

 cleft, there are but five open clefts. The first, second, and third 



arches are wholly devoid of branchial 

 filaments : the fourth and fifth support 

 each a biserial gill, while the sixth 

 arch retains only an anterior hemi- 

 branch, which, however, as the source 

 of its blood supply seems to indicate, 

 may consist of "emigrant" gill -fila- 

 ments from the posterior hemibranch 

 of the fifth arch. 2 Interbranchial septa 

 are practically non-existent, the flat- 

 tened leaf-like gill-lamellae being free 

 except at their attached bases, and thus 

 repeating a characteristic Teleostean 

 feature. A " hyoidean " hemibranch or 

 FIG. 166. The second branchial pseudobranch, supplied from the ventral 

 cleft of Neoceratodus, to show aorta, is present, but as the hyobran- 



the dorsal and ventral con- . . J 



tinuity of two hemibranchs on Chial clett IS closed it projects into the 



opposite sides of the same cleft, branchial cavity immediately in front of 



b.c, Branchial cleft ; b.f, bran- 

 chial filaments; g.r, gill- the cleft between the first and second 



Spencer.) (Fl ' m Baldwin branchial arches. In Lepidosiren* the 

 branchial arches are reduced to five and 



the clefts to four, the hyobranchial and fifth clefts being closed. 

 There is a " hyoidean " hemibranch resembling that of Protopterus. 

 The facts furnished by the study of the numerical and 

 structural variations in the gill-clefts, gills, and gill-arches of 

 different groups of Fishes prove that atrophy of these structures 

 takes place at opposite ends of the series. We have examples of 

 this anteriorly in the suppression of the hyo-mandibular cleft and 

 its hemibranch, and of the hyoidean hernibl-anch, as the result of 



1 Newton Parker, Trans. Hoy. Irish Acad. xxx. 1892, p. 161 ; Bridge, Trans. 

 Zool. Soc. xiv. 1898, p. 361. 



- Boas, Morph. Jahrb. vi. 1880, p. 345. See Fig. 201, p. 340. 



3 Bischoff, Lepidosiren -paradoxa, Leipzig, 1840 ; Hyrtl, Abhand. d. bb'hm. 

 Gesellsch. 1845, p. 637 ; also Bridge, op. cit. pp. 344, 345. 



