Animals, 1877), they really belong to the labial segment which has not become completely fused with the headcapsule.
The dorsal half of the collar is the cerebral commissure, the ventral the labial commissure.
This is shown by the labial commissure and pedal cords of the nervous system, by the opening of the gonad into the right kidney, and by other points.
The mouth parts are well developed, consisting of an upper lip, powerful mandibles, maxillae with three-jointed palpi, and a deeply quadrifid labium or lower lip with three-jointed labial palpi.
Maxillary and labial palpi are also present, and the latter, together with the labrum or lower lip, form the rostrum.
According to Spengel, the pair of ganglia near the mouth, variously called labial or cerebral, represent the cerebral pair and pleural pair of a gastropod combined, and the parietosplanchnic pair correspond to the visceral ganglia, the commissure which connects them with the cerebro-pleural representing the visceral commissure.
O, Posterior labial tentacle.
There is thus a tendency to assimilation, and instead of a guttural followed by a labial semi-vowel, a new labial consonant p is produced.
Lb, Hood-like appendage of the labial tentacle.
The labial commissure gives off a subradular commissure which also bears two ganglia, these being in close relation to a special sense-organ called the subradular organ, an epithelial projection with nerve-endings, lying in front of the radula and probably gustatory in function.