are also provided with a cochlea branchialis, to which, the organ described in Lutodeira, is, in its form, structure and uses, quite similar.
I may add that a few genera of the Salmones (Cuv.) or rather Characini (Müll.) viz. Prochilodus and Citharinus, also possess an accessory respiratory organ, well supplied with nerves from the Vagus; it is situated above their gill chambers, and is either a straight blind chamber, or has a curved sac-like form, in both cases receiving venous blood from the heart, and returning red blood to the base of the aorta. My friend, Professor Kner, a short time since, showed me the same organ in Anodus.
The following peculiarities in the structure of the gills of Lutodeira are unique, no other clupeoid or characine fish affording a trace of them:—
1st. Each interspace between the branchial arches is divided into a superior and inferior compartment by a short, strong and non-elastic ligament, which unites the articulations of the basi- and cerato-branchial bones (Owen) of each arch, with the like articulations of the same bones, opposite to them. The branchial arches therefore cannot be divaricated from one another, and their interspaces, the branchial clefts, cannot be so much dilated as in other fish, but they remain permanently in a state of extreme narrowness, and the current of water which passes through them, must necessarily be very small.
2nd. The cartilaginous combs, or fringes, attached to the concave borders of the branchial arches, are set in two rows on each arch; these two rows are likewise divergent, so that the tips of the fringes of the outside row of one arch, meet the tips of the fringes of the inside row of the next one. The tips of each pair of fringes firmly coalesce and cannot be separated without breaking them. Each branchial cleft is therefore bridged over by a succession of gothic arches, equal in number to the cartilaginous filaments in every fringe, and there is no free passage for the current of water. The water is, therefore, it may be said, filtered through the coalesced fringes, whose tips are directed towards the mouth, and, whatever may be the amount of heterogenous particles in the water, they must be with certainty caught between the pallisades, just as a fish is caught in a net; the surprising length, fineness and delicacy of the respiratory branchial lamellæ on the convex edges of the branchial arches, and the excessive richness of their capillary net-work of vessels, are such as fully to account for all these elaborate guards against mechanical injury to so frail an organism.
3. On a peculiar arrangement of the Gill chamber in Poly acanthus.
This fish, one of the Labyrmthidæ, presents a very peculiar arrangement of its gill chambers.
