A mouth (m), situated on the ventral surface, leads into an alimentary canal with a terminal anus (an). A præ-oral lobe is well developed, as in Bipinnaria, but there is no post-anal lobe. The bands of cilia have the same general form as in Bipinnaria. There is a præ-oral band, and a longitudinal post-oral band; and the two bands nearly meet at the apex of the præ-oral lobe ([fig. 273]). A contractile band passes from the œsophagus to the apex of the præ-oral lobe, and a diverticulum ([fig. 272], W) from the alimentary tract, directed towards the dorsal surface, is present. Contractile cells are scattered in the space between the body wall and the gut.
In the following stage ([fig. 274] A) a conspicuous transverse post-oral band of a single row of long cilia is formed, and the original bands become more sinuous. The alimentary diverticulum of the last stage becomes an independent vesicle opening by a pore on the dorsal surface ([fig. 274] A, w). The contractile cord is now inserted on this vesicle. Where this cord joins the apex of the præ-oral lobe between the two anterior bands of cilia a thickening of the epiblast (? a ganglion) has become established, and on it are placed two eye-spots ([fig. 273] oc, and [fig. 274] A). A deep bay is formed on the ventral surface of the larva.
Fig. 273. Young Tornaria.
(After Müller.)
m. mouth; an. anus; w. water-vascular vesicle; oc. eye-spots; c.c. contractile cord.
Fig. 274. Two stages in the development of Tornaria. (After Metschnikoff.)
The black lines represent the ciliated bands.
m. mouth; an. anus; br. branchial cleft; ht. heart; c. body cavity between splanchnic and somatic mesoblast layers; w. water-vascular vesicle; v. circular blood-vessel.