We imagined the gill in question to be the remnant of a gill fully formed in extinct Ganoid types, and therefore expected to find it better developed in the larva than in the adult. That the contrary is the fact appears to us fairly certain, although we cannot at present offer any explanation of it.
Systematic Position of Lepidosteus.
A. Agassiz concludes his memoir on the development of Lepidosteus by pointing out that in spite of certain affinities in other directions this form is “not so far removed from the bony Fishes as has been supposed.” Our own observations go far to confirm Agassiz' opinion.
Apart from the complete segmentation, the general development of Lepidosteus is strikingly Teleostean. In addition to the general Teleostean features of the embryo and larva, which can only be appreciated by those who have had an opportunity of practically working at the subject, we may point to the following developmental features[551] as indicative of Teleostean affinities:—
(1) The formation of the nervous system as a solid keel of the epiblast.
(2) The division of the epiblast into a nervous and epidermic stratum.
(3) The mode of development of the gut (vide pp. [752-754]).
(4) The mode of development of the pronephros; though, as shewn on p. [822], the pronephros of Lepidosteus has primitive characters not retained by Teleostei.
(5) The early stages in the development of the vertebral column (vide p. [779]).
In addition to these, so to speak, purely embryonic characters there are not a few important adult characters:—