4i6 



CEPHALOCHORDA. 



cirri. In the larva the hood is 

 absent, and the mouth is flush 

 with the surface. 



The mouth opens into the 

 pharynx, which, like it, is richly 

 ciliated. The pharynx, like that 

 of Tunicates, and indeed of Fishes 

 also, is modified for respiration 

 (Fig. 178, Ph.). Its walls are 

 perforated by numerous gill-slits 

 on each side, and between these 

 lie supporting bars alternately 

 split and unsplit at their lower 

 ends. 



Along the mid-dorsal and mid- 

 ventral lines there are grooves, 

 respectively called hyper- and 

 hypo-branchial. The latter is 

 comparable to the endostyle of 

 Ascidians, by which name it is 

 often called. As in Ascidians, 

 two ciliated bands — the peri- 

 pharyngeal bands — encircle the 

 anterior part of the pharynx. 



The water current which enters 



Fig. 179. — Development of atrial cham- 

 ber in Amphioxus. — After Lankester 

 and Willey. 



In I. the metapleural folds are seen sending 

 a slight projection inwards. In II. the two 

 projections have united and enclose a small 

 space (ATS), which is the rudiment of the 

 atrial chamber. In III. this space is enlarg- 

 ing at the expense of the body cavity, which 

 it pushes up before it. A comparison of 

 this figure with the cross-section of the 

 adult (Fig. 178) will show the relation of 

 coslom and atrial chamber. 



FR., coslomic space within dorsal fin ; AL., 

 gut ; S., coelomic space of metapleural fold ; 

 MP., metapleural fold; SAT., projection 

 which forms floor of atrial chamber ; AC, 

 aorta; B.C., body cavity ; S.I.V., sub-intes- 

 tinal vein ; N. , nerve-cord ; SH. , sheath of 

 notochord ; MY., myotome; C, remains 

 of myocffil ; AT., atrial chamber. The 

 dotted line indicates the mesodermic wall 

 of the body cavity. 



