The single pair of antennæ is formed in the next stage, and is followed by the remaining post-oral appendages, which arise in succession from before backwards somewhat later than the segments to which they appertain.
The posterior part of the embryo becomes uncoiled, and the whole embryo bent double in the egg ([fig. 168] B).
The mouth appears as a slit-like opening between and below the procephalic lobes. On each side and somewhat behind it there grows out an appendage—the first post-oral pair ([fig. 169], 1)—while in front and behind it are formed the upper and lower lips. These two appendages next turn inwards towards the mouth, and their bases become gradually closed over by two processes of the procephalic region ([fig. 170], m). The whole of these structures assist in forming a kind of secondary mouth cavity, which is at a later period further completed by the processes of the procephalic region meeting above the mouth, covering over the labrum, and growing backwards to near the origin of the second pair of post-oral appendages.
Fig. 170. Ventral view of the head of an embryo of Peripatus capensis at a late stage of development.
l. thickening of epiblast of procephalic lobe to form supra-œsophageal ganglion; m. process from procephalic lobe growing over the first post-oral appendage; o. mouth; e. eye; 1 and 2, first and second pair of post-oral appendages.
The antennæ early become jointed, and fresh joints continue to be added throughout embryonic life; in the adult there are at present fully thirty joints. It appears to me probable (though Mr Moseley takes the contrary view) from the late development of the paired processes of the procephalic lobes, which give rise to the circular lip of the adult, that they are not true appendages. The next pair therefore to the antennæ is the first post-oral pair. It is the only pair connected with the mouth. At their extremities there is formed a pair of claws similar to those of the ambulatory legs ([fig. 171]). The next and largest pair of appendages in the embryo are the oral papillæ. They are chiefly remarkable for containing the ducts of the slime glands which open at their bases. They are without claws. The succeeding appendages become eventually imperfectly five-jointed; two claws are formed as cuticular investments of papillæ in pockets of the skin at the ends of their terminal joints.
