ii. 7 



FEEDING OF AMPHIOXUS 



3i 



sensory tentacles, the velum. The oral hood contains a complex set of 

 ciliated tracts, the 'wheel organ' of Miiller, and this plays a part in 

 sweeping the food particles into the mouth (Figs. 8 and 9). Near its 

 centre is a groove, Hatschek's pit, formed as an opening of the left 

 first coelomic sac to the exterior (p. 44). 



The main operation of food collec- 

 tion is performed by the pharynx, a 

 large tube, flattened from side to side, 

 whose walls are perforated by nearly 

 200 oblique vertical slits, the number 

 increasing as the animal gets older. 

 The slits are separated by bars con- 

 taining skeletal rods and further sub- 

 division is provided by cross-bars 

 (synapticulae). Since the bars slope 

 diagonally many of them are cut in a 

 single transverse section, but it must 

 be remembered that they are essen- 

 tially the vertical portions of the main 

 walls of body and pharynx, where 

 these have not been perforated by a 

 gill-slit. Such a portion of the body 

 wall must contain a coelomic space 

 and this can in fact be seen in the 

 original or primary gill bars. How- 

 ever, an increase of the ciliary surface 

 is produced by downgrowth of secon- 

 dary or tongue bars from the upper 

 margin, dividing each primary slit; 



these secondary bars contain no coelom. The coelomic spaces in the 

 primary bars, of course, communicate above and below with con- 

 tinuous longitudinal coelomic cavities (Fig. 6). 



There are cilia on the sides and inner surfaces of the gill bars, the 

 lateral ones being mainly responsible for driving the water outwards 

 through the atrium and thereby drawing the feeding current of water 

 in at the mouth. In the floor of the pharynx lies the endostyle, con- 

 taining columns of ciliated cells, alternating with mucus-secreting 

 cells, which produce sticky threads in which food particles become 

 entangled. Various currents then draw the sticky material along until 

 it reaches the mid-gut. The frontal cilia of the gill bars produce an 

 upward current, driving the mucus from the endostyle into a median 



Fig. 9. Transverse section through 

 front end of amphioxus. 



b.c. buccal cirri; e. eyespot; H.p. Hats- 

 chek's pit; ?i. notochord; n.C. nerve-cord. 



