180 
The facts of comparative embryology are opposed to the homology. 
The lungs are paired structures in their origin, and they arise 
from the haemal region of the gut. The swimming bladder in its de- 
velopment is an unpaired organ, even if sometimes bilobed in the adult. 
It arises, except in Polypterus, from the neural side of the gut. The 
relations and non-relations respectively to branchial aortic arches also 
present insurmountable difficulties!). The case of Polypterus — a near 
ally of Lepidosteus — I do not consider worth discussion, until other 
arguments are set aside, nor can the opposite side draw conclusions 
from it, so long as the ontogeny is unknown. 
It will be noticed that I include the Marsipobranchii under 
those sub-classes which possess a swimming bladder. This step calls 
for an explanation. 
I am now brought to another section of this paper, viz. the re- 
lationships of the different families to one another. 
The relationships of Marsipobranchii and Ganoidei. 
The consideration of this subject shall be brief; for I propose to 
return to this difficult question at a future date. 
The conviction has been gradually forced upon me, as the out- 
come of my work on the two groups, that the Ganoidei and Mar- 
sipobranchii are more closely related than hitherto supposed. 
The nervous and urinogenital systems of the two sub-classes show 
very close resemblances, not only in the larva, but also in the adult. 
The same holds for the development and structure of the ovum, for 
the gastrulation and formation of the germinal layers, and for the 
development of the nervous system. 
On the surface the absence of an air-bladder in the Marsipo- 
branchii is a stumbling block, but perhaps the difference is more 
apparent than real. 
It is conceivable, that the peculiarities of the branchial region of 
Petromyzon might be caused by the inclusion of a swim-bladder, such 
1) Boas (Uber die Arterienbogen der Wirbeltiere. Morphol. Jahrb. 
Bd. XIII, 1887) has shown that the true pulmonary artery of the Di- 
pnoi, Amphibia and higher Vertebrates is formed from the sixth aortic 
arch. He expressly states that a true pulmonary artery is wanting in 
Lepidosteus and the Teleostei. ‘This must also be true of the other 
Ganoidei. 
