164 



SUB-PHYLUM CEPHALOCHORDA. 



by a membrane called the velum., 

 and is fringed by twelve velar 

 tentacles, which must not be 

 confused with the external 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 re- 

 spiration (Fig. 249, Ph.). Its 

 walls are perforated by numerous 

 gill-slits on each side, and be- 

 tween these lie supporting bars 

 alternately split and unsplit at 

 their low r er 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, 



FIG. 250. 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 {A T.\ which is the rudiment of the 

 atrial chamber. In III. this space is enlarg- 

 ing at the expense of the ccelom, which it 

 pushes up before it. A comparison of this 

 figure with the cross-section of the adult 

 (Fig. 249) will show the relation of ccelom 

 and atrial chamber. 



.#/?., ccelomic space within dorsal fin; AL. 9 

 gut ; S., ccelomic space of metapleural fold ; 

 ATP., metapleural fold ; SA Y\, projection 

 which forms floor of atrial chamber; AO., 

 aorta; B.C., ccelom; .9./.F., sub-intes- 

 tinal vein; A 7 "., nerve-cord; S7/. sheath of 

 notochord ; MY., myotome ; C., remains 

 of myoccel ; AT., atrial chamber. The 

 dotted line indicates the mesodermic wall 

 of the ccelom. 



