264 



THE PROTOCHORDATA. 



ns 



pinnae. An important function of these plumes is to 

 produce currents of water by the action of their cilia, 

 which vibrate in such a direction that the water with 

 food-particles is led into the mouth. The superfluous 

 water is led out from the proximal portion of the aliment- 

 ary canal by a single pair of gill-slits which are not visible 



in surface view, since they 

 are overhung by a fold of 

 the integument known as 

 the post-oral lamella or 

 operculum, corresponding to 

 the posterior free fold of 

 the collar in Balanoglossus 

 (Fig. 126). 



In its internal organisa- 

 tion, if due allowance be 

 made for its U-shaped ali- 

 mentary canal, Cephalodis- 

 cus greatly resembles Bala- 

 noglossus (Figs. 126, 127). 

 The buccal shield of the 

 former is obviously the 



Fig. 126. Longitudinal frontal (right 



and left) section through an adult Cephalo- equivalent of the probos- 

 discus. (After HARMER.) 



be 1 -. Second portion of body-cavity cis of the latter, and the 

 (collar-coelom). bc z . Third portion of . . . , 



body-cavity (trunk coelom). ^-.Pharynx. cavlt Y which it Contains 



c.p. Collar-pores, g.s. Gill-slits, int. in- corresponds to the probos- 



testine. n.s. Nervous system, op. Oper- 



cuium. oes. CEsophagus. st. stomach, cis-cavity. Moreover, the 



t. Base of tentacle. . . _ . . 



proboscis-cavity in Cephalo- 



discus (i.e. the cavity of the buccal shield) communicates 

 with the exterior by two proboscis-pores placed right and 

 left of the dorsal middle line. 



Following behind the buccal shield is the collar-region, 

 from which the branchial plumes arise dorsally, while 



