Fig. 87. Larva of Eurylepta auriculata immediately after hatching. Viewed from the side. (After Hallez.)
m. mouth.
The stages within the egg of this latter type agree precisely with those already described in Leptoplana. After the formation of the mouth the body elongates, remaining however cylindrical. A fold forms on the anterior side of the mouth, giving rise to a large upper lip. Two posterior processes are next formed, and other processes soon arise, constituting the whole of those found in the free larva. The embryo next shakes off its egg membranes by a series of vigorous contractions. When free it has the form represented in the annexed figure ([fig. 87]).
It is so similar to Müller’s ([fig. 88]) and Moseley’s larvæ that all three may be dealt with together.
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.