. Outlines of zoology. by a membrane called the velum,and is fringed by twelve velartentacles, which must not beconfused with the external the larva the hood is absent,and the mouth is flush with thesurface. The mouth opens into thepharynx, which, like it, is richlyciliated. The pharynx, like thatof Tunicates, and indeed ofFishes also, is modified for re-spiration (Fig. 249, /%.). Itswalls are perforated by numerousgill-slits on each side, and be-tween these lie supporting barsalternately split and unsplit attheir lower ends. Along the mid-dorsal and mid-ventral lines there are groove
. Outlines of zoology. by a membrane called the velum,and is fringed by twelve velartentacles, which must not beconfused with the external the larva the hood is absent,and the mouth is flush with thesurface. The mouth opens into thepharynx, which, like it, is richlyciliated. The pharynx, like thatof Tunicates, and indeed ofFishes also, is modified for re-spiration (Fig. 249, /%.). Itswalls are perforated by numerousgill-slits on each side, and be-tween these lie supporting barsalternately split and unsplit attheir lower ends. Along the mid-dorsal and mid-ventral lines there are grooves,respectively called hyper- andhypo- branchial. The latter iscomparable to the endostyle ofAscidians, by which name it isoften called. As in Ascidians,. Fig. 250.—Development of atiial cham-ber in Amphioxtis.—After Lankestcrand Willey. In I. the metapleural folds are seen sendinga slight projection inwards. In II. the twoprojections have united and enclose a smallspace {A T.\ which is the rudiment of theatrial chamber. In III. this space is enlarg-ing at the expense of the ccelom, which itpushes up belore it. A comparison of thisfigure with the cross-section of the adult(Iig. 249) will show the relation of coeloinand atrial chamber. FR.^ coelomic space within dorsal fin; AL.^gut; .y., coslomic space of metapleural fold ;MP.^ metapleural fold; SAT,^ projectionwhich forms floor of atrial chamber; AO.^aorta; ^ coilom ; ., sub-intes-tinal vein; N.^ nerve-cord; 6//., slieath ofnotochord; MY.^ myotome; C, remainsof myocoel;^ AT.^ atrial chamber. Iliedotted line indicates the mesodermic wallof the coelom. ^ODV CAVITY. 465 two ciliated bands—the peripharyngeal bands—encircle theanterior part of the pharynx. The water-current
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Keywords: ., bookcentury1900, bookdecade1910, bookidcu3192, booksubjectzoology
