Lophius piscatorius is one of the forms which, according to Hyrtl[558], is provided with a head-kidney only, i.e. with that part of the kidney which corresponds with the anterior swelling of the kidney of other types. For this reason I was particularly anxious to investigate the structure of its kidneys.
Each of these bodies forms a compact oval mass, with the ureter springing from its hinder extremity, situated in a forward position in the body-cavity. Sections through the kidneys shewed that they were throughout penetrated by uriniferous tubules, but owing to the bad state of preservation of my specimens I could not come to a decision as to the presence of Malpighian bodies. The uriniferous tubules were embedded in lymphatic tissue, similar to that which forms the anterior part of the apparent kidneys in other Teleostean types.
With reference to the structure of the Teleostean kidneys, the account given by Stannius is decidedly more correct than that of most subsequent writers. In the note already quoted he gives it as his opinion that there is a division of the kidney into the same two parts as in the Sturgeon, viz. into a spongy vascular part and a true secreting part; and on a subsequent page he points out the absence or poverty of the uriniferous tubules in the anterior part of the kidney in many of our native Fishes.
Prior to the discovery that the larvæ of Teleosteans and Ganoids were provided with two very distinct excretory organs, viz. a pronephros or head-kidney, and a mesonephros or Wolffian body, which are usually separated from each other by a more or less considerable interval, it was a matter of no very great importance to know whether the anterior part of the so-called kidney was a true excretory organ. In the present state of our knowledge the question is, however, one of considerable interest.
In the Cyclostomata and Amphibia the pronephros is a purely larval organ, which either disappears or ceases to be functionally active in the adult state.
Rosenberg, to whom the earliest satisfactory investigations on the development of the Teleostean pronephros are due, stated that he had traced in the Pike (Esox lucius) the larval organ into the adult part of the kidney, called by Hyrtl the pronephros; and subsequent investigators have usually assumed that the so-called head-kidney of adult Teleosteans and Ganoids is the persisting larval pronephros.
We have already seen that Rosenberg was entirely mistaken on this point, in that the so-called head-kidney of the adult is not part of the true kidney. From my own studies on young Fishes I do not believe that the oldest larvæ investigated by Rosenberg were sufficiently advanced to settle the point in question; and, moreover, as Rosenberg had no reason for doubting that the so-called head-kidney of the adult was part of the excretory organ, he does not appear to have studied the histological structure of the organ which he identified with the embryonic pronephros in his oldest larva.
The facts to which I have called attention in this paper demonstrate that in the Sturgeon the larval pronephros undoubtedly undergoes atrophy before the adult stage is reached. The same is true for Lepidosteus, and may probably be stated for Ganoids generally.
My observations on Teleostei are clearly not sufficiently extensive to prove that the larval pronephros never persists in this group. They appear to me, however, to shew that in the normal types of Teleostei the organ usually held to be the pronephros is actually nothing of the kind.
A different interpretation might no doubt be placed upon my observations on Lophius piscatorius, but the position of the kidney in this species appears to me to be far from affording a conclusive proof that it is homologous with the anterior swelling of the kidney of more normal Teleostei.