4 6 



ASCIDIANS 



Endostyle. The mucus just referred to is produced in the 

 long canal-shaped gland called the endostyle or hypobranchial 

 groove, which runs along the entire ventral edge of the 

 branchial sac (Fig. 18, end). The sides, and especially the floor 

 of the endostyle, are richly ciliated, while there are four (or six) 

 strongly -marked, peculiarly -shaped glandular tracts, two (or 

 three) on each side (Fig. 20, gl} running along its length, and 

 separated by areas of closely-packed fusiform cells with short 

 cilia, amongst which are found some bipolar sensory cells. 



This organ corresponds to the hypopharyngeal groove of 



FIG. 20. Transverse section of the endostyle of Ascidia mentula, x 350. bl.s, Blood - 

 sinus ; end.l, lips of the endostyle ; fjl, 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. 



Amphioxus and the median part of the thyroid gland of Verte- 

 brata. 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 appear- 

 ance 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 collect- 

 ing the food particles, it passes backwards along the dorsal edge 



