The metamorphosis consists in the whole animal growing longer and flatter, and in the arms becoming gradually shorter and shorter till they finally disappear altogether, and the larva acquires the ordinary adult form.
The lobed larval form of the Turbellaria has some points of resemblance to the Pilidium form of nemertine larva described below, yet its resemblance to this interesting larva is less close than would appear to be the case with certain turbellarian larval forms recently described by Götte and Metschnikoff, which are in some respects intermediate in character between the larva of Leptoplana and those just described.
The observations of Götte (No. [184]) were made on Planaria Neapolitana and Thysanozoon Diesingi, and those of Metschnikoff (No. [188]) on Stylochopsis ponticus. The larvæ of all these forms undergo more or less of a metamorphosis, but the accounts of their development are not easily reconciled.[91] The early stages of Planaria are like those of Leptoplana, as described by Keferstein. Four large hypoblast cells become surrounded by small epiblast cells, which commence to be formed on the dorsal side. The hypoblast cells divide and arrange themselves in two bilaterally symmetrical rows. A small blastopore is left by the small cells on the ventral surface, which communicates with an otherwise closed and ciliated cavity which is formed between the two rows of hypoblast cells. The blastopore would seem to remain permanently open, and to be placed at the base of a deep pit, lined by epiblast cells, which constitutes the stomodæum.
Fig. 89. Planarian larva (probably Planaria angulata). (From Agassiz.)
Fig. 90. Planarian larva (probably Plamaria angulata). (From Agassiz.)
The embryo now becomes dorsally convex, while the ventral surface becomes marked with a median furrow and grows out laterally into two lobes, and anteriorly into a ventrally-directed upper lip. The whole surface becomes ciliated, and the cilia are especially prominent on the ventral processes and the summit of the dorsal dome. A bunch of strong cilia becomes formed in front of the dome, and a less marked bunch behind. The larva is now stated by Götte closely to resemble a Pilidium. It soon, however, extends itself, and the two bunches of cilia become situated at the anterior and posterior extremities of the body. The ventral processes become inconspicuous prominences of the side of the body. Götte believes that the larva undergoes no further metamorphosis.