Art. X.—On the New Zealand Lamprey.
[Read before the Philosophical Institute of Canterbury, 6th November, 1901.]
The lamprey has been found in New Zealand in three well-marked stages of growth—the Ammocœtes, the larval form, very similar to the corresponding stage in the European lamprey; the adult Geotria, with a well-developed gular pouch; and the Velasia, a form intermediate between the other two, with no gular pouch. Little is known of its life-history or habits; but in October the Velasia come up some of the rivers in shoals, and are caught by the Maoris for food, and the Maoris say that they come down again in December with gular pouches. Very few specimens of any stage have hitherto been preserved, but recently a large number of Velasia were sent to us alive from the Mataura River, up which they were making their annual migration, and as regards this stage we were able to work from the fresh material.
Both the Velasia stage and the adult Geotria were first described by Gray in 1851, and were classified by him as distinct genera (Geotria and Velasia) of the family Petromy-zonidœ. Gunther, in 1870, ranks the two forms as separate species of the genus Geotria; the pouched form he calls Geotria australis, and the Velasia he calls Geotria chilensis, since Geotria in the Velasia stage was first discovered in Chili. Recently Ogilby, in reclassifying the Australian lampreys, reverts to Gray's system of classification, and places the two forms in different genera.
Before minutely examining the animals themselves it had occurred to us that possibly, since the larval Ammocœtes was formerly regarded as a separate genus, a similar mistake had been made in the case of the Velasia, which might be only an intermediate form (since it was only found in New Zealand, Australia, and Chili, where the Geotria was also found), and that, if Velasia and Geotria actually were distinct species, it
was impossible to account for some forms which have been found intermediate between the Velasia stage and the adult. Günther himself, in speaking of one such form, suggests that Velasia may possibly develope a gular pouch later in life, in which case the distinction between the two forms would be doubtful. Ogilby, however, as already noted, actually reverts to the idea of a generic difference between the two. The distinctions upon which the old classifications were based are merely external ones—the shape and size of the oral disc, the position of the teeth, the presence or absence of a gular pouch, and the shape and position of the fins.
In the Velasia stage the head is small, the oral disc is round and small, and the teeth are closely packed together in rows, whilst in the adult Geotria the head region is enormously developed, the oral disc being very large and flattened on the lower margin, owing to the growth of the gular pouch below it; and the teeth, which, as we have ascertained by careful examination, correspond in number and position to the teeth of the Velasia form, are some distance apart, owing to the growth of the disc between them. The gular pouch is, of course, only fully developed in the adult Geotria, but intermediate forms have been found possessing a slight gular pouch.
There is no very great difference between the fins of the Velasia and the adult. They are larger and situated relatively farther forward in the Velasia, but they change gradually with the growth of the animal, and we have a series of four specimens which exhibit the different conditions of the fins at different stages in life.
Fourteen specimens of the Velasia which were dissected were found to be sexually immature, males and females, whilst the only two pouched forms which we have dissected are sexually mature, or nearly so. None of the former observers appear to have examined the internal anatomy at all, but have drawn their conclusions from the external differences, probably because of the scarcity of material at their disposal.
In other respects the Velasia closely resembles the adult, but is longer and thinner. We could not compare the living forms, as we have not yet been able to obtain a fully grown Geotria alive, and we find by experiment that spirit-preserved specimens undergo considerable shortening.
As the two generic names Velasia and Geotria have been applied to the same animal, we have had to decide which to retain, and, following Gunther's nomenclature, we propose to call the adult form Geotria australis, and to use the term “Velasia” to distinguish the intermediate form, just as the term “Ammocœtes” is still used to distinguish the larva.
Thus it appears that, whereas the northern lampreys of the genus Petromyzon undergo only one metamorphosis—namely, from the Ammocœates to the adult—the southern form (Geotria) undergoes two well-marked changes, from the Ammocœtes to the Velasia, and then from the Velasia to the adult, which latter represents a further stage in development never reached by the northern forms.