20 



ANATOMY OF AMPHIOXUS. 



guarded against the intrusion of noxious substances. 

 Everything entering the mouth has to pass through a 

 vestibule richly provided with sensitive epithehal cells. 

 This vestibule consists of the oral hood with its marginal 

 cirri, at the back of which lies the definite oral opening or 

 velum, as it was called by Huxley on account of its 

 resemblance to a similar structure in the young lamprey 

 (Ammocoetes). (Cf. Fig. 3.) In the adult the velum 

 carries twelve tentacles of its own, the velar tentacles, 

 which are not to be confused with the buccal cirri of the 

 oral hood. The velar tentacles project in a backward 

 direction freely into the pharynx. 



Fig. 6. — A. Portion of a buccal cirrus to show groups of sense-cells. 

 B. Isolated cells of the skin ; two columnar sense-cells carrying a sensory hair 

 and one cylindrical epidermic cell with striated cuticular border. (After Lan- 



GERHANS.) 



Groups of sense-cells occur on the side of the buccal 

 cirri at intervals (Fig. 6). Some of these cells bear a 

 vibratile cilium at their free ends, and others bear stiff 

 hairs. Both kinds of cells are mingled in the same group. 



