546 MULLER ON THE STRUCTURE OF THE GANOIDS, 
pearance of the latter, a systemic artery is formed; in the second 
case, a systemic artery is formed from a branch of the gill-veins 
carrying bright red blood, i. e. from the artery of the pseudo- 
branchia, by the disappearance of the latter. The pseudo~ 
branchia, which is wanting in Scymnus, I have found in an early 
foetal stage*. It may here be asked whether Polypterus and 
Spatularia do not, in the foetal state, possess the opercular gill 
of Acipenser, Scaphirhynchus and Lepisosteus, which are formed 
on the general plan of the Ganoids, and if the equivalent which 
we have found may not be produced by a reduction of the oper- 
cular gill? If it be not so, nevertheless enough has been done 
here on the general plan of the Ganoids in the relation which 
we have described, and which differs from all the osseous fishes. 
As the last trace of the opercular gill is preserved in the oper- 
cular branch of the gill-artery in Polypterus and Spatularia, so 
we find a trace of the blowing-hole in Lepisosteus. As such | 
consider a blind depression in the palate internally to the pseudo; 
branchia ; in some individuals it penetrates deeper, and forms a - 
narrow canal, in the same manner as occurs in those sharks 
which have no open blowing-hole, the Carcharias. As this 
canal is open in the foetus of Carcharias, the same may be ima- 
gined to exist in the earliest stages of Lepisosteus. The blind 
canal also exists in the palate of Scaphirhynchus. In my pre- 
vious memoir I was compelled, for want of materials, to leave the 
relation of the vessels of the opercular gill to those of the pseudo- 
branchia obscure; they are now perfectly cleared up. The for- 
mer obtains its blood from the gill-artery, the gill-vein of the 
respiratory opercular gill becomes converted into the artery of 
the operculum. This proceeds externally to the articulation of 
the os hyoides with the temporal bone, then runs along the 
inner side of the operculum, and gives off the pseudo-branchial 
artery. The pseudo-branchial vein becomes the carotis interna. 
It is thus certain that the opercular gill in Lepisosteus has.a 
respiratory function, as in the Sturgeons, but that the other 
accessory gill is either a pseudo-branchia or rete mirabile, and 
in fact a rete mirabile caroticum, as occurs in the Plagiostomi 
and Sturgeons, as I have already stated in my first treatise. 
Lepisosteus has two carotids, an external and an internal; the 
internal only of these arteries stands in the relation we have 
mentioned to the pseudo-branchia as rete mirabile. The carotis 
* Abhandl. d. Akad, d. Wissensch. xu Berlin for 1840, p. 252. 
