In the cod and whiting the pseudobranchia is situated, as Mr. Simon has described it, near the upper extremity of the first branchial arch on each side. It lies in a recess which is bounded by the gill below, and above by the outer extremity of that transverse fold of mucous membrane which limits the extent of the palate. Its structure is peculiar; Mr. Simon regards it as a thyroid ; but from this opinion I feel obliged to differ. The following description is taken from examination of the organ in the Cod, but applies equally to that in the Whiting. It is a body of light red colour, semi-transparent aspect, flattened so as to present two faces, about one line and a half in thickness, and having two borders, one convex and slightly notched, the other somewhat concave. V The surface is slightly uneven or nodulated. It is enclosed in a cap sule, through which some large vessels are seen ramifying. Its general aspect is that of "glandular flesh," and certainly not of a mere congeries of vessels. In structure it appears to consist of parallel folds of homogeneous membrane, beneath which is spread a vascular plexus, and which are covered by an unusually developed epithelium. This epithelium appears under the form of granular cells of an oval, circular, or irregular form, not distinctly nu cleated, and having a diameter of inch. These do not appear to constitute a mere in vestment, but to form a layer of some thick. ness, ' filling up the intervals between the adjacent processes or folds of homogeneous membrane. These organs, manifestly con structed after the type of gills, evince thus a tendency to assume a glandular structure ; yet I can see no sufficient reason for sup posing them to represent the thyroid, from which they differ so entirely in structural characters. In the whiting I could detect no trace of a sublingual gland, nor of the small posterior one; but in the Eel, where the pseudobranchia was absent, I found, by the aid of the microscope, between the first and second basibranchials, a small mass, which consisted in great part of fat, but contained also some large vesicles closely resembling those of a real thyroid. Their diameter varied from to to inch. They had an ho
mogeneous envelope lined by an epithelium, consisting of small non-nucleated pellucid cor puscles, and surrounding a cavity filled by a transparent somewhat refracting fluid.
In a Plearonectid I found the pseudobranchia quite free, uncovered by mucous membrane, and projecting a series of small distinct leaf lets into the bronchial cavity. It differs essen tially from that of the cod or whiting, and showed no tendency to assume a solid glan dular form, but was manifestly a real though minute gill.
In a Carp I found the pseudobranchia with some difficulty. It was situated very deeply between the anterior part of the upper ex tremity of the first branchial arch, and the posterior border of the pterygoid muscle. Its structure was entirely that of a gill, consisting of parallel folds of a membrane, arranged transversely to a median axis, and overlaying a vascular plexus. These folds were covered with a kind of scaly epithelium, which was often detached in large pieces, and mingled with some circular cells, closely resembling mucous globules. I could find no trace of the sublingual nor of other adjacent gland.
From the few facts now detailed I think it may be concluded, (I) That there is no evi dence from their structural characters to prove that the pseudobranchia- are the representa tive of the thyroid. (2) That in some in stances organs which seem to be of a totally different kind are found, which resemble very closely the structure of the thyroid when it unquestionably exists. Mr. Simon lays stress upon the circumstance, that the pseudobranchim and the sublingual gland re ceive their vascular supply from the same source, viz. the 1st branchial vein ; this, how ever, is not constantly the case, the pseudo branchim in osseous fishes, according to Pro fessor Owen, serving only as diverticula to the ophthalmic, and not to the cerebral circulation; so that on this ground they cannot he sup posed to have similar functions. For a philo sophical discussion, however, of the analogy and homology of the pseudobranchia to the thyroid, I would refer to Professor Owen's Lectures, vol. ii. p. 270. note.