304 



THE LANCELET. PHYLUM CHORDATA 



not on the sides of the body, beat outwards, and so maintain 

 a current of water in at the mouth, down the pharynx, and out 



m./. ,P^- «»<• al.fi: 



"^ 





S^Ss^S^gj^^^^ggss^'^^^:^ 



Fig. 22S.—Branchiostoma, from the ventral side, after the floor of the atrium 



has been cut open. 

 For references to illustration, see p. 299. 



through the slits to the atrium and so to the exterior by the 



atriopore (Fig. 230). The frontal cilia beat upwards. On the floor 



of the pharynx Hes the endostyle, 



which consists of four rows of 



ciliated cells alternating with four 



rows of mucus-secreting cells. Food 



particles become entangled in the 



mucus, and the frontal cilia, which 



beat upwards, convey it to the roof 



of the pharynx, where there is a 



deep median epipharyngeal groove, 



with cilia which beat backwards. 



,n.c. 



inch. 



myc. 



/^mym. 



, d.f, r 





ygon 



Fig. 229. — A view from the left side of the 

 region around the atriopore of a 

 specimen of Branchiostoma with the 

 atrial floor expanded. 



at.f.. Atrial floor ; at.p., atriopore ; d.f.r., dorsal fin ray ; 

 gon., gonads ; myc, myocommata ; mynt., myomeres; 

 n.c, nerve cord ; nch., notochord ; v.f.r., ventral 

 fin ray. 



Fig. 230. — The outlines 

 of a transverse section 

 through the pharyngeal 

 region of Branchiostoma 

 with arrows to show 

 the course of currents 

 in the pharynx and 

 atrium. Heavy arrows 

 show the main current 

 passing from pharynx 

 to atrium, light arrows 

 show the currents 

 which transport to the 

 epipharyngeal groove 

 mucus containing the 

 particles retained in 

 the pharynx. 



The feeding of the lancelet is thus very similar to that of the 

 mussel (p. 275). Such feeding on small organic particles is called 

 microphagy. A short distance in front of the atriopore the 



