264 



THE PROTOCHORDATA. 



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 

 opcrcuhtniy 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 

 Fig. 126.— Longitudinal frontal (right former is obviously the 



and Ieft)section through an adult Cephalo- equivalent of the proboS- 

 discus. (After HarMER.) ' ^ 



of the latter, and the 

 cavity which it contains 

 corresponds to the probos- 

 cis-cavity. Moreover, the 

 proboscis-cavity in Cephalo- 



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



with the e.xterior 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 



bc^. Second portion of bodv-cavitv CIS 

 (collar-coslom). bfi. Third portion of 

 body-cavity (trunk coelom). br. Pharynx. 

 c.p. Collar-pores, g.s. Gill-slits, int. In- 

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

 culum, oes. CEsophagus. st. Stomach. 

 /. Base of tentacle. 



