Rhabdocœla. The development of some of the Rhabdocœla has recently been studied by Hallez. The ova are mostly laid in capsules, one in each capsule. Sometimes the development commences before the capsules are laid, at other times not till afterwards. In certain forms (Mesostomum) there are summer eggs with thin capsules which develop within the parent, while hard capsules, forming what are known as winter eggs, are laid in the autumn, and the embryo hatched in the spring.
The ova of the Rhabdocœla like those of the fresh-water Dendrocœla are enveloped in yolk elements derived from the vitellarium.
The segmentation probably takes place in the same way as in Leptoplana. A stage with four equal cells has been observed by Hallez, and there is subsequently an epibolic gastrula. The embryo becomes ciliated while still within the capsule and, according to Hallez, the pharynx arises as a bud of the hypoblast. The proboscis in Prostomum originates as an epiblastic invagination.
Nemertea.
Some Nemertea develop without and some with a metamorphosis.
The most remarkable type of Nemertine development with a metamorphosis is that in which the ovum develops into a peculiar larval form known as Pilidium, within which the perfect worm is subsequently evolved. Closely allied to this type is one in which the sexual worm is developed within a larval form as in Pilidium, but in which the larva has no free-swimming stage, and is therefore without the characteristic appendages of the Pilidium. This is known as the type of Desor and is confined (?) to the genus Lineus. The Pilidium and the Desor type may be first considered (vide Barrois, No. [192]).
The type of Desor. The segmentation is regular and leads to the formation of a blastosphere with a large segmentation cavity. The blastosphere is converted by invagination into a gastrula ([fig. 91] A). The blastopore is soon carried relatively forwards by the elongation backwards of the archenteron, and, according to Barrois, actually forms the mouth. Owing to the elongation of the archenteric cavity the embryo assumes a bilateral form ([fig. 92] A) in which the dorsal and ventral surfaces can be distinguished, the mouth (m.) being situated on the ventral surface.
Fig. 91. Three stages in the development of Lineus.
(After Barrois.)