The eggs of the Discophora, each enclosed in a delicate membrane, are enveloped in a kind of mucous case formed by a secretion of the integument, which hardens into a capsule or cocoon. In each cocoon there are a limited number of eggs surrounded by albumen. The cocoons are attached to water plants, etc. In Clepsine the embryos leave the cocoon very soon after they get rid of the egg membrane, but in Nephelis they remain within the cocoon for a very much longer period (27‑28 days after hatching). The young of Clepsine, after their liberation, attach themselves to the ventral surface of their parent.
Our knowledge of the development of the Discophora is in a very unsatisfactory state; but sufficient is known to shew that it has very many points in common with that of the Oligochæta, and that the Discophora are therefore closely related to the Chætopoda. In Clepsine there is an epibolic gastrula, and mesoblastic bands like those in Euaxes are also formed. In Nephelis however the segmentation is very abnormal, and the formation of the germinal layers cannot easily be reduced to an invaginate gastrula type, though probably it is modified from such a type. Mesoblastic bands similar to those in the Oligochæta occur in this form also.
The embryology of Clepsine, which will serve as type for the Leeches without jaws (Rhyncobdellidæ), has recently been studied by Whitman (No. [365]), and that of Nephelis, which will serve as type for the Leeches with jaws (Gnathobdellidæ), has been studied by Bütschli (No. [359]). The early history of both types is imperfectly known[144].
Formation of the layers.
Clepsine. It is necessary to give a full account of the segmentation of Clepsine, as the formation of the germinal layers would be otherwise unintelligible.
Segmentation commences with the division of the ovum into two unequal spheres by a vertical cleavage passing from the animal to the vegetative pole. By a second vertical cleavage the large segment is divided into two unequal parts, and the small one into two equal parts. Of the four segments so produced three are relatively small, and one, placed at the posterior end, is large. Each of the four segments next gives rise to a small cell at the animal pole. These small cells form the commencement of the epiblast, and, according to Whitman, the mouth is eventually placed in their centre. Such a position for the mouth, at the animal pole, is extremely unusual, and the statements on this head require further confirmation.
Fig. 158. Two views of the larva of Clepsine. (After Whitman.)
o. oral extremity; m. mouth; pr. germinal streak.