“Gaimardia trapesina (Gould).
Modiola trapesina (Lamarck), Anim. sans Vert., vii. 24. Delessert; Recueil, pl. 13, f. 7.
ANIMAL with the mantle closed, except a small aperture at its anterior third, for the passage of the foot, a circular siphonal opening posteriorly, destitute of fringes, and a small excretory orifice just above and behind the posterior adductor muscle. Edges of the mantle yellow, destitute of any fringes or lobes. Branchiæ ample, the outer pair two-thirds the size of the inner, which cover nearly the whole space beneath the mantle, rounded at their free anterior end, and narrowing backwards; attached along the middle of the abdominal mass through their entire length. Mouth rather large, some distance above the base of the foot, with a short compressed, labial appendage on each side. Foot linguiform, and capable of great extension, with a groove on the outer or ventral side; at its base are two convex lobes, which are emarginate on the posterior side; between, and at the junction of these lobes, at the emarginate part, is a conspicuous orifice, from whence proceeds the byssus, and which leads to a cavity occupying all the posterior portion of the foot, seeming to communicate with the visceral mass, by a small posterior puncture. Stomach large, extending across the liver; intestinal canal not traced, but the rectum appeared to traverse the heart, and to terminate just behind it; the heart itself is attenuated at both ends. Striæ on the branchiæ very apparent. Liver chocolate-coloured, reticulated by numerous white fibres, divided into four lobes, two anterior and superior ones, separated by a deep longitudinal fissure, and two lateral and larger ones of an oval form; behind these, of a creamy colour and consistence, are the ovaries, constituting two-thirds of the whole abdominal mass. The two adductor muscles are large for the shell.
The animal moves with considerable rapidity, as follows: the foot is doubled, so that its point is brought to its base; it is then extended to great length, carrying a byssal thread, and attaching it to the object towards which it is moving; by a sudden contraction, the shell is jerked forwards the length of the thread; and this operation is repeated until the desired spot is reached, when it is there anchored by a number of threads thrown out in various directions. These threads are very elastic, and are found to proceed from the cavity at the base of the foot.
On some specimens the branchiæ were found to have the intervals of the striæ filled with minute, but perfectly formed shells, those nearest the margin being dark-brown, while those towards the dorsum were paler and nearly white. [J. P. C.]
It will be perceived that the animal of this shell varies from that of Mytilus, in having the mantle closed, except at three orifices. On the other hand, their interior structure, and especially the apparatus and manner of locomotion, correspond precisely. As the animal differs from that of Mytilus in characters which are considered of generic value, it is proposed to give the name of GAIMARDIA, after M. Gaimard, who in all probability, was the first to observe it as an object of scientific interest, and who has also done so much else for science.
Figure 568, lateral view of the shell and animal; 568a, dorsal view in outline; 568b, ventral margin, showing the opening of the mouth; 568c, the gills; 568d, the viscera; 568e, two views of the foot; 568f, mouth and palpi.
SHELL ovate-trapezoidal, thin and fragile, ventricose, covered by a smooth, shining epidermis, of a pale olive-colour usually, but occasionally bright yellow, orange, dark brown, and almost black; beaks near the anterior end, prominent, antrorse, contiguous; anterior end compressed, sloping abruptly to the base; dorsal margin nearly rectilinear; posterior margin regularly rounded; ventral margin somewhat sinuous and gaping anteriorly, forming a small elliptical passage for the foot and byssus; the right valve has two small, oblique, cardinal teeth, which receive two corresponding ones of the opposite valve; muscular impressions quite distinct, united by a somewhat sigmoid pallial impression. Substance of the shell purplish or chestnut-coloured externally; yellowish towards the margin; interior always deep purple within the pallial impression, and rosy white outside of it.
Length seven-tenths of an inch; height nine-twentieths of an inch; breadth one-fourth of an inch.
Found in latitude 47° 30’ S., long. 59° 50’ W., adhering to the giant focus seen floating in these regions, and abounds on kelp Orange Harbour, where some were found an inch and a half in length.
This seems clearly to be the shell described by Lamarck without a locality, and figured by Delessert; but as it is a curious shell, and no full description of it is to be found, it is given here. It would seem to be the same species alluded to in the “Voyage of the Astrolabe,” (Zool. iii. 216), as “a Modiola rare and perhaps new, which attaches itself to the focus, a short distance at sea,” near the Falkland Islands.”
(Gould, 1852)
