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.) A is a side view in optical section. B and C are two later stages from the ventral (oral) surface. _ae._ archenteron; _sc._ segmentation cavity; _hy._ hypoblast; _me._ mesoblast; _ep._ epiblast; _m._ mouth; _st._ stomach; _pr.d._ prostomial disc; _po.d._ metastomial disc; _pr._ proboscis.] [FIG. 92. THREE STAGES IN THE DEVELOPMENT OF LINEUS.] (After Barrois.) A. Side view of an embryo at a very early stage as an opaque object. B and C. Two late stages, seen as transparent objects from the ventral surface. _ae._ archenteron; _m._ mouth; _pr. d._ prostomial disc; _po.d._ metastomial disc; _cs._ lateral pit developing in B as a diverticulum from the oesophagus; _pr_. proboscis; _ms._ muscular layer (?); _ls._ larval skin about to be thrown off; _me._ mesoblast; _st._ stomach.]