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Updated: June 26, 2025
If now the tongue be forcibly drawn forward, a curved ridge may be seen behind it. The throat is lined with mucous membrane covered with ciliated epithelium, which secretes a lubricating fluid which keeps the parts moist and pliable. An excess of this secretion forms a thick, tenacious mass of mucus, which irritates the passages and gives rise to efforts of hawking and coughing to get rid of it.
Carapace smooth, rather convex, and with three keels above; the beak, longly produced, ending in a spine, simple on the side and produced into a keel on each side behind; the central caudal lobe rather narrow, indistinctly divided in half, and like the other lobes flexile at the end, the lateral lobes with a central keel ending a slight spine; the hands elongated, compressed, smooth, with a thickened, toothed, inner margin, which is ciliated above; wrist with two conical spines on the inner side.
But between the two germinal layers a mesoderm is developed, a soft mass of connective tissue, in which the organs are embedded. The Turbellaria are still represented by a number of different forms, in both fresh and sea-water. The surface is covered with ciliated epithelium, a stratum of ectodermic cells.
If a Bipinnaria, a Brachiolaria, a Pluteus, is competent to produce the Echinoderm, which is so widely different from it; if a hydroid polype can produce the higher Medusa; if the vermiform Trematode 'nurse' can develop within itself the very unlike Cercaria, it will not appear impossible that the egg, or ciliated embryo, of a sponge, for once, under special conditions, might become a hydroid polype, or the embryo of a Medusa, an Echinoderm."
Carapace smooth, rather convex, and with three keels above; the beak, longly produced, ending in a spine, simple on the side and produced into a keel on each side behind; the central caudal lobe rather narrow, indistinctly divided in half, and like the other lobes flexile at the end, the lateral lobes with a central keel ending a slight spine; the hands elongated, compressed, smooth, with a thickened, toothed, inner margin, which is ciliated above; wrist with two conical spines on the inner side.
A division of labour resulting from such a variation being advantageous, and tending therefore to increase in descendants, would end in a differentiation like that shown in the gemmules of various low types of Metazoa, which, ovate in shape, are ciliated over one part of the surface only.
The upper part of the antheridium becomes separated from the parent stem by a septum, and its contents are converted into ciliated motile antherozoids.
At the foremost end of it there is a small black pigment-spot, a rudimentary eye; and a narrow canal leads to a superficial sense-organ. In the vicinity of this optic spot we find at the left side a small ciliated depression, the single olfactory organ. There is no organ of hearing.
The inhalent pores are very minute, and open into small subdermal cavities which communicate by means of interradial tubes with the ciliated chambers, the latter being very small ramifications of the interradial channels, and in them the movement causing the current of water is maintained.
From the moneron he proceeds to the amoeba a simple cell, with a kernel, which still corresponds to the egg of man in its first state. To the fourth stage he assigns the planæa, corresponding to the embryonic development of an albumen and the planula or ciliated larva.
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