It is regarded as a modified zooecium, the polypide of which has become vestigial, although it is commonly represented by a sense-organ, bearing tactile hairs, situated on what may be termed the palate.
It is terminated by a well-developed structure (fg) corresponding with the apical sense-organ of ordinary Trochospheres, and an excretory organ (nph) of the type familiar in these larvae occurs on the ventral side of the stomach.
This is followed by the atrophy of many of the larval organs, including the brain, the sense-organ and the ciliated ring.
Near the base of the ctenidium is a patch of sensory epithelium innervated from the branchial nerve, forming a sense-organ called the osphradium, whose function is to test the water entering the branchial cavity.
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.