The works of Francis Maitland Balfour, Volume 2 (of 4) : $b A treatise on comparative embryology: InvertebrataBalfour, Francis M. (Francis Maitland)
Science
The works of Francis Maitland Balfour, Volume 2 (of 4) : $b A treatise on comparative embryology: Invertebrata
Balfour, Francis M. (Francis Maitland)
Embryology; Zoology
The body is somewhat oval, and slightly pointed behind. At the
anterior end are placed the eyes, two in the youngest larva of Müller,
and twelve in the older larva (fig. 88), and in the middle of the
ventral surface is the mouth. It is surrounded by a strong fold, and
leads into an alimentary canal, which is at first simple, but in the
older larvæ is much branched. A bilobed ganglion connected with two
nerve cords is placed anteriorly. The superficial epithelium is
ciliated, and below it is a layer of cells (cutis) derived from the
primitive epiblast, in which are formed the usual rods (Hallez). The
chief peculiarity of the larva consists in the presence of elongated
processes covered with long cilia, and so connected together by a
ciliated band that the whole together forms, in Müller's larva at any
rate, _a_ _lobed præoral ciliated band_ (fig. 88). This band is not
quite so clear in Hallez' figures. Müller's youngest larva was
provided with eight very long lobes; three were dorsal, viz. a median
anterior, and two lateral placed far back; three ventral, viz. a
median in the front of the mouth forming a large upper lip, and two
processes at the sides of the mouth. The number was completed by two
lateral processes of the body. All the processes except the dorsal
median one are shewn in fig. 88. In Hallez' larva, fig. 87, the six
posterior processes form a rather definite ring, while one flagellum
projects from the front end of the body immediately below the eyes,
and a second flagellum behind. In Moseley's youngest larva six
processes only were present, though subsequently eight became formed
as in Müller's larvæ.
[FIG. 88. MÜLLER'S TURBELLARIAN LARVA (PROBABLY THYSANOZOON). VIEWED
FROM THE VENTRAL SURFACE. (After Müller.)
The ciliated band is represented by the black line.
_m_. mouth; _u.l._ upper lip.]
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.
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