24 J. H. ORTON. 



lateral view of a gill-bar or a gill-filament, shown in Fig. 4, scattered cilia 

 can be seen on the atrial epithelium. These cilia appear to lash in the 

 direction shown by the arrow on the atrial side of the bar, and by com- 

 parison with a gill-filament of a Lamellibranch, may be called ab-frontal 

 cilia. They probably help in a small way in producing the main current 

 and also in cleaning the atrial surface of the gill-bar. 



DIRKTION IN WHICH LATERAL CILIA LASH 



PHARYNX / DIRECTION IN W HICH FRONTA L CILIA LASH / ^.fc. -^^ 



VENTRAL /■> ■'^''*^*'*^«5%w!*?j,<!^!s*s»ts4o'/»«p'^^^ 

 END 



■ DORSAL 





ATRIUM 



Fig. 4.* — Side view of a single living primary gill-bar of Amphioxus, showing the ciliary 

 mechanisms. 



The direction in which the lateral cilia, I.e., lash, to produce the main current 

 is shown by the large arrows crossing the gill-bar. 



The direction in which the frontal cilia, f.c, lash is indicated by the arrow above 

 these cilia. The true direction in which these particles transport cilia is along the 

 face of the gill-bar and upwards towards the reader. 



The ab-frontal cilia, ab.f.c, appear also to lash in a similar direction to that 

 of the frontal cilia as is shown by the accompanying arrow. 

 I.e. Lateral row of cilia, 



f.c. I Frontal or pharyngeal row of ciHa. 



ab.f.c. Scattered cilia on the atrial or ab-frontal face of the gill-bar. 

 Ph. Pharyngeal side of gill-bar. 



Atr. Atrial or jieri- branchial side of gill-bar. 

 V. Ventral end of gill-bar. 



tr. Connexions joining the gill-bar to others. 



CILIATION OF THE ENDOSTYLE. 

 There still remains for examination the ciliation on the endostyle. 

 Under a low power in such a view as Fig. 3, particles suspended in the 

 water may be observed to be lashed rapidly out of the endostyle on to the 

 face of the gill-bars in the direction denoted by the small arrows in the 

 middle of the figure (i.e. on the endostyle). Examination of the endo- 

 style under a high power brings out the presence of three main sets of 

 cilia, two outer or lateral sets and one median. The cilia on these lateral 

 rows are short and lash rapidly across the length of the endostyle. These 

 are the cilia which throw particles out of the endostyle on to the gill-bars. 

 The median cilia on the endostyle are long and appear only to shake with 



* I am indebted to Mr. L. R. Crawshay for the lettering in this figure, and also for 

 that in Figures 2, 3, and 5. 



f The short cilia figured by Benham (15, Plate 6) have not yet been seen in the 

 living filament, although they have been carefully looked for. Further observations, 

 hov/ever, will be made on this point. 



