86 FISHES CHAP. 
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' 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 
Frc. 166.—The second branchial PSeudobranch, supplied from the ventral 
a genet Siew one oe eect ea 
tinuity of two hemibranchs on Chial cleft is closed it projects into the 
Se eee oe Soe branchial cavity immediately in front of 
chial filaments; g.7, gill. the cleft between the first and second 
eS (From 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 hemibranch, as the result of 
1 Newton Parker, Trans. Roy. 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. 
® Bischoff, Lepidosiren paradoxa, Leipzig, 1840; Hyrtl, Abhand. d. bohm. 
Gesellsch, 1845, p. 637 ; also Bridge, op. cit. pp. 344, 345. 
