Fig. 15. A Parapodium of Tomopteris.
(From Gegenbaur.)
o. Collection of germinal epithelial cells lining the body cavity.
Discophora.
(36) H. Dorner. “Ueber d. Gattung Branchiobdella.” Zeit. f. wiss. Zool., Vol. XV. 1865.
(37) R. Leuckart. Die menschlichen Parasiten.
(38) Fr. Leydig. “Zur Anatomie v. Piscicola geometrica, etc.” Zeit. f. wiss. Zool., Vol. I. 1849.
(39) C. O. Whitman. “Embryology of Clepsine.” Quart. J. of Micr. Sci., Vol. XVIII. 1878.
The ovary of the Discophora is formed of a mass of cells enveloped in a membranous sack. In Branchiobdella there is placed in the central axis of these cells a column of nucleated protoplasm from which the cells themselves are budded off. The development of the ovum takes place by the enlargement, etc. of one of the peripheral cells, which eventually bursts the wall of the sack and is freely dehisced into the body cavity.
In most other Leeches (except Piscicola and its allies) there is found a more specialized arrangement of the same nature as in Branchiobdella. There are one or more coiled egg-strings which lie freely in a delicate sack continuous with the oviduct. Each egg-string is formed of a central rachis and of a peripheral layer of cells[22]. The ova are formed by the enlargement of the peripheral cells accompanied by a deposition of food-yolk. Food-yolk appears to be formed in the rachis even more energetically than in the protoplasm of the ova. When ripe the ova fall into the ovarian sack.
In Piscicola the development of the ovum is somewhat peculiar but resembles in certain respects that of Bonellia (p. [45]). The ova are developed from the primitive germinal cells which fill up the ovarian sack. The nuclei in these cells increase in number, and a nucleated peripheral layer of each cell becomes separated from the central part, which also contains nuclei. This latter part next divides into numerous cells, of which one eventually forms the ovum, and the remainder constitute a mass of cells adjoining it as in Bonellia ([fig. 16]). This mass of cells eventually disappears, and is probably employed in the nutrition of the ovum.
The ovaries of the Leech appear to belong to the tubular type in that the ova are not formed from part of the epithelium lining the body cavity; but if, as seems probable, the true affinities of the Leeches are with the Chætopoda, the investment of the ovaries must be of a secondary nature. It should be noted that the ova are not, as in the ordinary tubular ovary, developed from the epithelium lining the ovarian tube.