STRUCTURE AND DEVELOPMENT OP LEPIDOSTEUS. 
385 
Embryology,’ by one of us,* will not fail to recognise that the folds of the retina at 
the sides of the choroid slit, and the mesoblastic process passing through this slit, are 
strikingly similar in Lepidosteus and Elasmobranchii; and that, if we are justified in 
holding them to be an imperfectly-developed processus falciformis in the one case, we 
are equally so in the other. 
Johannes Muller mentions the absence of a processus falciformis as one of the 
features distinguishing Ganoids and Teleostei. So far as the systematic separation of 
the two groups is concerned, he is probably perfectly justified in this course; but it is 
interesting to notice that both in Ganoids and Elasmobranchii we have traces of a 
structure which undergoes a very special development in the Teleostei, and that the 
processus falciformis of Teleostei is therefore to be regarded, not as an organ peculiar 
to them, but as the peculiar modification within the group of a primitive Vertebrate 
organ. 
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. 371) a passage from Agassiz’ memoir showing 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’ state¬ 
ments, 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 21, figs. 11 and 13, and Plate 24, figs. 40 and 44. 
The result of our examination has been to show that the disc is provided with a series 
of papillae often exhibiting a bilateral arrangement. The papillae are mainly con¬ 
stituted 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 unpro¬ 
vided 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 papillae to pour out a sticky secretion. 
* Vol. ii., p. 414. 
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