On the ventral wall of the branchial sack there is formed a narrow fold with thickened walls, which forms the endostyle. It ends anteriorly at the stomodæum and posteriorly at the point where the solid remnant of the archenteron in the tail was primitively continuous with the branchial sack. The whole of the alimentary wall is formed of a single layer of hypoblast cells.
A most important organ connected with the alimentary system still remains to be dealt with, viz. the atrial or peribranchial cavity. The first rudiments of it appear at about the time of hatching, in the form of a pair of dorsal epiblastic involutions ([fig. 8] V. kl), at the level of the junction between the brain and the spinal cord. These involutions grow inwards, and meet corresponding outgrowths of the branchial sack, with which they fuse. At the junction between them is formed an elongated ciliated slit, leading from the branchial sack into the atrial cavity of each side. The slits so formed are the first pair of branchial clefts. Behind the first pair of branchial clefts a second pair is formed during larval life by a second outgrowth of the branchial sack meeting the epiblastic atrial involutions ([fig. 8] VI. 1ks and 2ks). The intestine at first ends blindly close to the left atrial involution, but the anus becomes eventually formed by an opening being established between the left atrial involution and the intestine.
During the above described processes the test remains quite intact, and is not perforated at the oral or the atrial openings.
The retrogressive metamorphosis of the larva.
The development of the adult from the larva is, as has already been stated, in the main a retrogressive metamorphosis. The stages in this metamorphosis are diagrammatically shewn in [figs. 10] and [11]. It commences with the attachment of the larva ([fig. 10] A) which takes place by one of the three papillæ. Simultaneously with the attachment the larval tail undergoes a complete atrophy ([fig. 10] B), so that nothing is left of it but a mass of fatty cells situated close to the point of the previous insertion of the tail in the trunk.
Fig. 10. Diagram shewing the mode of attachment and subsequent retrogressive metamorphosis of a larval Ascidian. (From Lankester.)
The nervous system also undergoes a very rapid retrogressive metamorphosis; and the only part of it which persists would seem to be the dilated portion of the spinal cord in the trunk (Kupffer, No. [28]).
The three papillæ, including that serving for attachment, early disappear, and the larva becomes fixed by a growth of the test to foreign objects.