34 J. H. ORTON. 



gill-bars and the cilia on the papillae on the bars. These cilia lash across 

 the length of the pharynx from the endostyle towards the dorsal lamina, 

 but are not specially active nor specially powerful in the simple 

 Ascidians, and food-collectioii is accelerated by the waving of the 

 longitudinal bars in a transverse direction. In this way the food 

 masses are pushed on as well as lashed onwards towards the dorsal groove. 

 The papillae of the gill-bars thus assist in food-collection, and in those 

 animals in which they point towards the dorsal lamina, as in Ascidia 

 mentula, act somewhat like the bristles of a brush in pushing food masses 

 onwards. In Ascidia mentula cilia have been observed on the atrial 

 surface of the gill-bars and the atrial epithelium of the peri-branchial 

 wall ; doubtless these are present also in other Ascidians, and have the 

 function of cleaning the walls of the atrium. 



The cilia on the outer or lateral portions of the endostyle lash from 

 the endostyle groove on to the pharynx, and in this way doubtless function 

 mainly in transferring mucus secreted by the endostyle on to the walls of 

 the pharynx. These cilia, moreover, are probably seldom required to throw 

 particles on to the gill, for the tendency of the main stream will be to pass 

 towards the middle of the pharynx where the main pull on the water is 

 being effected by the lateral cilia. Herdman (9, p. 17) has estimated the 

 number of gill openings in an adult Ascidia mentula of medium size to be 

 about 192,000, and as there are rows of lateral cilia on each side of these 

 slits, there are in all about 384,000 short rows of cilia lashing water 

 through the pharynx of the animal ; the main direction of the current 

 thus produced (see Figs. 7 and 8) is away from the endostyle. It is 

 therefore curious that this groove can ever have been regarded as an 

 important food-collecting organ. On the outer edge of the endostyle 

 of Ciona there are definite short transverse grooves in which the cilia 

 are specially powerful, and lash from the endostylar groove on to the 

 pharynx. 



The median ciliated tract of the endostyle bears cilia up to two milli- 

 metres in leng-th. Roule (8) describes the cilia as having a very slow 

 movement directed along the length of the groove and consisting of a 

 series of undulations beginning at the base of the cilia. I have not yet 

 been able to detect any movement in these cilia, and am therefore in 

 some little doubt as to their function. Since, however, food-particles are 

 not transported in the endostylar groove, these cilia do not appear to 

 have any transporting function. This view is further supported by 

 the fact that transporting cilia in Gastropods, Lamellibranchs (see 5, 

 'passim), and Amphioxus are always relatively very short and vibrate 



