[Illustration: FIG. 20.—Transverse section of the endostyle of _Ascidia mentula_, × 350. _bl.s_, Blood-sinus; _end.l_, lips of the endostyle; _gl_, glandular tracts; _i.l_, internal longitudinal bar; _l.v_, interstigmatic vessels; _m_, mantle; _p.br_, peribranchial cavity; _sg_, stigmata; _v.bl.s_, ventral blood-sinus.] This organ corresponds to the hypopharyngeal groove of Amphioxus and the median part of the thyroid gland of Vertebrata. It is interesting to notice that the (at least) four longitudinal tracts of gland-cells are of remarkable constancy, being found not only in all groups of Tunicata, including even the pelagic, tailed Appendicularians, but also in Amphioxus and in the young thyroid gland of the Ammocoete. When, in Ascidians, a third marginal glandular tract is added it has a different appearance from the two characteristic tracts. The mucus is carried forward by the action of the large floor-cilia of the endostyle (Fig. 20) to the groove between the peripharyngeal bands, and after encircling the anterior end of the branchial sac and collecting the food particles, it passes backwards along the dorsal edge {47}of the branchial sac to the oesophagus, guided by a membranous fold, the dorsal lamina (Fig. 21, _d.l_), which is more or less ridged or corrugated, and may be armed with marginal tags or even replaced by larger processes (the "languets") in some species of Ascidians. In the living animal the lamina has its free edge curved to the right hand side in such a manner as to constitute a fairly perfect tube along which the train of food passes. [Illustration: FIG. 21.—Antero-dorsal part of pharynx in _Ascidia mentula_, × 15. _br.s_, Part of branchial sac; _d.l_, dorsal lamina; _d.t_, dorsal tubercle; _p.br.z_, prebranchial zone; _p.p_, peripharyngeal bands; _sph_, sphincter of branchial aperture; _tn_, tentacle.]