b. Mouth Funnel, Mouth And Teeth.
As in Macrophthalmia, the margin of the mouth funnel carries an incomplete circle of cirri externally and a complete circle of fringed processes internally. (Fig. 8). The number of cirri varies from fifteen to twenty-two, eighteen being a common number. They are arranged in a horse-shoe shape, the gap being ventral. Two of
the laterally placed cirri (tentacles) one on each side, are more deeply pigmented and larger, and hence more striking to the eye than the others.
The number of fringed processes is about sixty. A drawing of one is given (Fig. 9.).
The labial teeth are arranged in rows radiating outwards (Fig. 8), except ventrally, where is present only a single row of teeth of rectangular shape. The labial teeth are shed at intervals—frequently it is easy to prize off one with a needle, when below is found another formed to take its place.
The supra-oral lamina carries four cusps—two broad outer ones and two more sharply-pointed inner ones. (Figs. 8 and 10).
Lahille (1915, Fig. 4) figures the supra-oral dentary lamina of his Geotria chilensis. In the attached legend he says “Entre las puntas medianas se ve un tercer par de puntas muy pequeñas, rudimento del par desarrollado en Caragola, género en el cual las dos
Fig. 8.—Velasia stage. Mouth funnel and Mouth. × circa 1¼. Externally the incomplete circle of cirri, internally the complete circle of fringed process.
SO, supra-oral lamina; IO, infra-oral lamina. The ant. lingual lamina with three cusps shows between it and infra-oral lamina a papillated fold of soft tissue, on each side the pad (Polster) P.
Fig. 9.—Velasia. Fringed process in profile.
mitades de la placa supraoral son aisladas y trifurcadas.” (Between the middle points is seen a third pair of very small points, rudiments of the like developments in Caragola (Mordacia), a genus in which the two halves of the supra-oral plate are isolated and trifurcated). In my Geotria specimens I have not been able to make out distinctly “a third pair of very small points,” though I have looked carefully for them.
The infra-oral lamina has a cuspidate margin. (Figs. 8 and 11). The number of cusps varies from ten to thirteen. Usually I find that the laterally placed cusps are more pronounced than the central ones, and sometimes the indications of centrally placed cusps are very faint. I have not seen regularly a large central cusp, as is figured by Lahille (1915). But there appears to be some variation as to the number and size of these cusps.
Both supra-oral and infra-oral horny laminae are shed.
The lingual armature consists of an anterior lamina and a pos-
terior pair of laminae. There are three cusps on the anterior lamina, as in Macrophthalmia, and further the middle cusp when well developed is not in the same plane as the other two but in a plane nearer the ventral surface also as in Macrophthalmia. (Figs. 8, 12 and 13). It is not, however, now longer than the other two. During this stage a series of changes takes place, changes which lead to the disappearance of the middle cusp. These changes have been observed and noted by Lahille (1915). With successive sheddings it appears, that the middle cusp becomes smaller and smaller, till finally only a little hillock is left between the two large ones to indicate its position. Figs. 14–16 will illustrate this. Fig. 15A represents the anterior lingual lamina of a Velasia. The horny outer husk or cap was easily prized off with a needle, when below was discovered a replacing structure (Fig. 15B), but with the middle cusp very much reduced and only represented by a hillock. Fig. 14 represents another condition frequently found in the Velasia—the middle cusp as long as the other two but extremely thin and delicate.
Fig 16 shows still another condition. Here three little hillocks were noted—it appeared as though with successive sheddings the middle cusp had migrated dorsally or postero-dorsally, so that it is now, though of course much reduced, in the same plane as the other two.
It is clear then, that this anterior lingual lamina is shed. Plate (1902) and Lahille (1915) have noted this. Behrend's statement (1892) quoted in Bronn (1905–24) to the effect that the “Gabelzahn” of Geotria is never shed appears therefore to be erroneous.
There are two posterior lingual laminae, each roughly triangular. The inner margin of each is cuspidate and there are four cusps on each margin. Replacing laminae with cusps are formed under these.
Beween the infra-oral lamina and the anterior lingual lamina is a transverse fold or soft papillated tissue. On each side this is continuous with a pad, “Polster” of Plate. These two pads are situated, one on each side of the pharynx, and at times appear very vascular. Between anterior and posterior lingual laminae is another papillated fold—“blumenkohlartigen Krause” of Plate. In preserved specimens between these two folds, the anterior lingual lamina may be often hidden from view.
Behrends (1892) has investigated the structure of the outer lingual lamina in Geotria. He names it the forked tooth. “Die Gestalt des Zahnes ist die einer Gabel mit zwei Zinken deren Spitzen leicht nach der Unterflache der Zunge zu gebogen sind. Zwischen den beiden langen Zacken befindet sich noch eine kleine, schlanke Hornspitze (Figs. 1 and 2 HS), so dass das ganze Gebilde den Eindruck einer Fischgabel macht und ich den obigen Namen dafür wählte.” As I have indicated before, the outer lingual lamina in Geotria does not possess originally the form described above, i.e., two lateral prongs (cusps) with a smaller slender middle prong. The original form, as exhibited in the Macrophthalmia and early Velasia stages, shows three equally developed cusps, the central one not in quite the same transverse plane as the other two. (Figs. 5 and 13). But after the Velasia has entered the rivers, this horny lamina or
cap with three equal cusps is shed and then a replacing lamina below it is revealed, which shows however only two large cusps and a much smaller central one, i.e., it is a “Gabelzahn.” It is probable there may be two or three sheddings before the central cusp reaches its greatest reduction. (Figs. 12, 13, 14, 15A and B, 16). When the central cusp has reached its greatest reduction (Gabelzahn), it appears that no more replacing structures are formed under this outer lingual lamina.
Fig 10.—Velasia. Supra-oral lamina. The scale on this figure applies also to Figs. 11–17.
Fig. 11.—Infra-oral lamina.
Fig. 12.—Ant. lingual lamina from before.
Fig. 13.—Ant. lingual in side view.
Fig. 14.—Ant. lingual with attenuated central cusp.
Fig. 15A.—Ant. lingual.
Fig. 15B.—The same after outer horny cap has been removed.
Fig. 16.—Another ant. lingual.
Fig. 17.—Post lingual laminae.
In the specimen (or specimens) which Behrends possessed, the outer lingual lamina was already at the “Gabelzahn” stage. He investigated this structure histologically and found no replacing lamina below. Since, however, he was unaware that at an earlier
stage this lamina possessed three equal cusps and was certainly subject to sheddings then, the conclusions to which he comes, based on examination of only “Gabelzahn” stages, are erroneous. “Dieser Verhornungsprocess, … … geht bei dem Gabelzahne während des ganzen Lebens vor sich, so dass es bei diesen Horngebilden niemals zur Entstehung einer zweiten Verhornungsschicht kommt, wie wir es sonst bei den anderen Zähnen von Geotria und denjenigen von Petromyzon zu finden gewohnt sind.”
Here a contrast is drawn between the outer lingual lamina and the other dental laminae of Geotria; and again “Der Gabelzahn wird nie gewechselt. Die Hornsubstanz, welche durch den Gebrauch des Zahnes abgenutzt wird, findet ihren Ersatz durch die dem Zahne anliegende Stachelschicht. Bei den übrigen Zähnen findet der Zahnersatz durch die Einleitung einer zweiten Hornbildung in einer tieferen Epidermisschicht statt.” This contrast does not exist. Replacing laminae are formed under the outer lingual laminae, as they are formed under the other laminae. This I have observed in Macrophthalmia and Velasia stages. But in the history of the outer lingual lamina there comes a time, i.e., the time when the middle cusp has reached its greatest reduction, when no more replacing laminae are formed. This is the “Gabelzahn” and it was from such a stage that Behrends drew his conclusions. The stage in which no more replacing laminae are formed under the outer lingual lamina is the stage approaching sexual maturity, a stage further in which it is highly probable the animal does not feed. The fact that replacing laminae cease to form here in this stage we may regard as part of the process “ďatrofia inerenti alla maturità sessuale.” (Cotronei 1927A).
Just behind the margin of the circular lip, the skin of the body is very loose. If an incision be made in the skin, an extensive subdermal space is opened into. This space is largest ventrally just behind the margin of the lip, where later the pouch will appear. But it extends laterally up to the level of the eyes and can be followed posteriorly as far as the first gill pore, growing less capacious as it goes. It is crossed by strands of connective tissue. Dorsally in this region is a similar sub-dermal space, which extends back as far as the nasal aperture. It is crossed by a transversely placed fold of connective tissue. The skin, then, in this region is firmly attached to the underlying tissue along two lines, laterally placed, at about the level of the eyes. The reason for the development of such extensive sub-dermal spaces in this region would appear to be this. When a lamprey is at rest, i.e. not sucking, the sides of the lip are folded in towards the middle line and the skin posterior to the lip does not appear folded or wrinkled, but smooth. When the lamprey sucks on to an object, the lip is expanded to its greatest transverse diameter and pressed hard up against the object, and the skin just behind the lip is seen to be thrown into folds and wrinkles. The development of this sub-dermal space is probably then to be correlated with the habit of sucking on to foreign objects. In other parts of the body the skin is firmly attached to the underlying tissues. In the Velasia stage indications of the commencing pouch may sometimes be observed. In the specimen figured, an
immature female taken at the mouth of the Manawapou River, Taranaki, there was no external indication of a pouch. (Fig. 22).