464 
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, 2%.) Its 
walls are perforated by numerous 
gill-slits on each side, and be- 
tween 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, 
Fic. 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 11, the two 
projections have united and enclose a small 
space (A7.), 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. 
FR., celomic space within dorsal fin; AZ., 
gut; S., coelomic space of metapleural fold ; 
4A7P., metapleural fold; SA7., projection 
which forms floor of atrial chamber; AQ., 
aorta; £&.C., celom; S$./.V., sub-intes- 
tinal vein; 4., nerve-cord; S//., sheath of 
notochord; JZ¥., myotome; C., remains 
of myocel; AZ., atrial chamber. ‘The 
dotted line indicates the mesodermic wall 
of the ccelom. 
