Fig. 94.
A. Pilidium with an advanced nemertine worm.
B. Ripe embryo of the Nemertea in the position it occupies in Pilidium. (Both after Bütschli.)
œ. œsophagus; st. stomach; i. intestine; pr. proboscis; lp. lateral pit; an. amnion; n. nervous system.
A ventral germinal plate is thus established, which gradually grows round the intestine of the Pilidium to form the skin of the future Nemertine. The outer thin layer of each of the discs grows pari passu with the inner layer, and furnishes an amnion-like covering for the embryo which is forming within the Pilidium ([fig. 94], an).
In connection with the young vermiform Nemertine there is formed on each side an outgrowth from the œsophagus ([fig. 94]) which is eventually placed in communication with the exterior by a ciliated canal[93]. The proboscis arises as an hollow invagination at the point where the two anterior discs fuse in front.
When the young Nemertine has become pretty well formed within the Pilidium it becomes ciliated, begins to move, and eventually frees itself and leads an independent existence, leaving its amnion in the Pilidium which continues to live for some time.
The central nervous system ([fig. 94]) is developed either before or after the detachment of the young Nemertine, according to Metschnikoff as a thickening of the epiblast. The young Nemertine is at first without an anus.
The development of the Nemertine within the Pilidium is clearly identical with that of the Lineus embryo within the larval skin; the formation of an amnion in the Pilidium constituting the only important difference which can be pointed out between the modes of origin of the young Nemertine in the two types.
So far as is known the forms which develop in a Pilidium, or according to the type of Desor, all belong to the division of the Nemertines without stylets in the proboscis, known as the Anopla.