CILIARY MECHANISMS ON THE GILL IN AMPHIOXUS. 25 



a wave-like motion which begins at the attached end. Particles may in- 

 deed be observed to rest on these cilia for some time, but such particles are 

 eventually worked towards the lateral sets of cilia, which soon transport 

 them to the gill-bars. In the passage of the particles from the median 

 to the lateral cilia on the endostyle one can often make out that the 

 particles have become embedded in mucus. It is therefore doubtless the 

 function of the median cilia on the endostyle to pass on mucus secreted 

 by the endostyle to the lateral endostyle cilia and thence to the gill-bars. 

 This mucus, along with that doubtless secreted also by the pharyngeal 

 epithelium of the gill-bars, serves to entrap food-particles and render the 

 transportation of these easier. 



Between the median and lateral sets of cilia on the endostyle a narrow 

 ciliated groove can be made out on either side (see Fig. 3). Particles are 

 frequently caught in these grooves, but they can be seen to be passed 

 quickly on to the lateral endostyle cilia and generally on to those portions 

 of the endostyle overlying a primary bar, and thence to the gill-bars. 



Before passing on to a recapitulation of the' ciliary mechanisms and 

 their function in Amphioxus, it will be convenient to examine the ciliary 

 arrangements so well known in the anterior end of the animal, namely 

 those in the buccal cavity, the wheel organ, and Hatchek's pit, and the 

 peri-pliaryngeal bands. 



THE FUNCTION OF THE WHEEL OEGAN, AND THE 

 PEEI-PHAKYNGEAL BANDS IN AMPHIOXUS.* 



The action of these organs can be made out by examining the anterior 

 end of the living animal through a microscope while it is feeding. During 

 the act of feeding the buccal tentacles are kept folded over one another. 

 In this way these tentacles act as a sieve, allowing only the finer suspended 

 particles to pass on into the buccal cavity. The efficiency of the buccal 

 tentacles in straining, when held in this manner, is increased by the 

 presence along the sides of the tentacles of conical papillso which sub- 

 divide the spaces between the tentacles, as is shown in Fig. 5. In this 

 manner the larger particles carried along in the ingoing current become 

 arrested on the buccal tentacles, only the finer particles being allowed to 

 pass onwards. A selection of the finer food-particles is thus effected. 



From the buccal tentacles the main current with the suspended 

 particles passes through the buccal cavity, the mouth and onwards into 

 the pharynx. 



In the buccal cavity, however, some particles fall out of the main 

 stream as a result of the occurrence of slack waters in the periphery of 

 * See Appendix on page 45 for an account of Andrew's work on feeding in Amphioxus. 



