14 



minute cilia which causes the regular current of water to 

 flow in at the branchial aperture and out at the atrial. 



The branchial sac is very large, much the largest organ 

 of the body (it may be 15 cm. in length), and extends 

 almost to the posterior end of the body, while the rest 

 of the alimentary canal lies upon its left side imbedded in 

 the body-wall. The food particles, consisting of very 

 minute plants and animals, are carried in through the 

 branchial aperture by the current of water, but most of 

 them do not pass out into the atrium, being caught by 

 the ciliary action of the peripharyngeal bands, and entangled 

 in the viscid substance which fills the groove between 

 them near the anterior end of the branchial sac. This 

 viscid substance, or mucus, is formed in a long canal- 

 shaped gland called the Endostyle or hypobranchial groove 

 (PI. II., figs. 2 and 4, end.), which lies along the ventral 

 edge of the branchial sac, and terminates both anteriorly 

 and posteriorly in a short cul-de-sac. 



The Endostyle. 



On the floor of the endostylar groove is found a tract 

 of cells with very long cilia (PI. II. , fig. 2) . On each side are 

 at least two (in some Ascidians three) laterally placed 

 clumps of gland cells (PI. IV., fig. 5), each clump separated 

 from its neighbours by an area of closely packed fusiform 

 cells with short cilia, amongst which are found some 

 sensory bipolar cells. The lips of the endostyle are formed 

 of ciliated cubical epithelium. 



This organ, on account of its thick glandular walls, 

 has an opaque appearance, and seems, in side view, to 

 run like a conspicuous solid rod in the more transparent 

 walls of the branchial sac — ^hence its name endostyle. It 

 is, however, really a gland, and corresponds to the hypo- 

 pharyngeal groove of Amphloxiis and the median part of 



