[520] Vol. II. p. 414 [the original edition].
Suctorial Disc.
One of the most remarkable organs of the larval Lepidosteus is the suctorial disc, placed at the front end of the head, to which we have made numerous allusions in the first section of this memoir.
The external features of the disc have been fully dealt with by Agassiz, and he also explained its function by observations on the habits of the larva. We have already quoted (p. [755]) a passage from Agassiz' memoir shewing how the young Fishes use the disc to attach themselves firmly to any convenient object. The discs appear in fact to be highly efficient organs of attachment, in that the young Fish can remain suspended by them to the sides of the jar, even after the water has been lowered below the level at which they are attached.
The disc is formed two or three days before hatching, and from Agassiz' statements, it appears to come into use immediately the young Fish is liberated from the egg membranes.
We have examined the histological structure of the disc at various ages of its growth, and may refer the reader to Plate 34, figs. 11 and 13, and Plate 37, figs. 40 and 44. The result of our examination has been to shew that the disc is provided with a series of papillæ often exhibiting a bilateral arrangement. The papillæ are mainly constituted of highly modified cells of the mucous layer of the epidermis. These cells have the form of elongated columns, the nucleus being placed at the base, and the main mass of the cells being filled with a protoplasmic reticulum. They may probably be regarded as modified mucous cells. In the mesoblast adjoining the suctorial disc there are numerous sinus-like vascular channels.
It does not appear probable that the disc has a true sucking action. It is unprovided with muscular elements, and there appears to be no mechanism by which it could act as a sucking organ. We must suppose, therefore, that its adhesive power depends upon the capacity of the cells composing its papillæ to pour out a sticky secretion.
Muscular System.
There is a peculiarity in the muscular system of Lepidosteus, which so far as we know has not been previously noticed. It is that the lateral muscles of each side are not divided, either in the region of the trunk or of the tail, into a dorso-lateral and ventro-lateral division.
This peculiarity is equally characteristic of the older larvæ as of the adult, and is shewn in Plate 41, figs. 67, 72, and 73, and Plate 42, figs. 74-76. In the Cyclostomata the lateral muscles are not divided into dorsal and ventral sections; but except in this group such a division has been hitherto considered as invariable amongst Fishes.