The sensory vesicle, larval brain and caudal nerve-cord are entirely destroyed by phagocytes at the metamorphosis. The adult brain arises as a proliferation from the hinder end of the sub-neural ("hypophysial") duct, and the viscera are first in nervated after its formation. The caudal part of the neural canal naturally disappears with the tail at metamorphosis, but the pharyngeal portion appears to persist in certain cases as the "rapheal" or "visceral" cord. The neural canal accordingly gives rise to two independent nervous systems, one after the other, for larva and adult respectively, as well as to ciliated funnel, duct, and gland in front of them.
In Enterogona the atrium arises in the embryo as a pair of ectodermal invaginations, and their dorso-lateral apertures usually remain separate until fixation, when they unite dorsally to form the cloacal siphon. Two pairs (rarely one only) of simple gill slits now arise, either simultaneously (Enterogona) or succes sively (Pleurogona). Normally the pharynx persists in this state over the larval period, but after fixation it expands greatly, and rapidly increases its fenestration. The various modes are re ducible to the remarkable process exemplified in Ciona. Each primary perforation elongates ventrally, turns back on itself at its lower end, and divides into two at this point. The short limb extends dorsally until two equal and parallel "protostigmata" have been produced from the original single perforation. This is the evidence for regarding the gill-slits of Ascidians as essentially similar to those of Amphioxus. Here, however, by a modification of the growth-process, the U-shaped form is attained by the gill slit extending round the tongue-bar, instead of the tongue-bar growing down into the slit. Although the rows of stigmata ulti mately produced may be very numerous, no Ascidian ever shows more than 3 pairs of these potential U-shaped slits, for with in creased size the later fenestration of the pharynx is produced by the subdivision of the 6 rows of stigmata first formed.
The process just described undergoes considerable abbrevia tion in most types. In Stolidobranchs, other than Molgulids, the primary perforations elongate into protostigmata, but do not re curve ; and in all Haplobranchs even protostigmata cease to appear as such, owing to the precocity of synapticulation.
It is highly characteristic of Tunicata that their buds possess a certain complexity from the outset of their formation. In Ascidiacea the ectodermal envelope of the bud, continuous with that of the parent, includes from the start an inner vesicle, de rived as an outgrowth from some internal organ. The inner
vesicle gives rise to all the organs of the bud except the outer most skin with its covering of test. It usually becomes trilobed, the lateral lobes developing into atrial chambers, the middle lobe into pharynx, gut, pericardium and nervous system. But the original inner vesicle has a different origin in the two primary groups of Ascidians—in Enterogona from the endodermal pharynx, through the intermediation of the epicardium, and in Pleurogona directly by outgrowth from the outer peribranchial (ectodermal) wall. Thus two modes of budding are distinguish able, viz., epicardial (or pharyngeal) and parietal (or peri branchial). The former alone is associated with the formation of a stolon, the apex of which is provided by a ventral prominence of the larval body beset with three "suckers" for fixation. After fixation (e.g., Clavelina) the larval body rotates on this ventral prominence, through a horizontal to an upright position, and the epicardium enters the stalk thus produced, dividing with it in all its subsequent ramifications. In Haplobranchia the intestinal loop also usually extends into the stalk, and the dilatation of this to form the abdominal cavity causes the epicardium to be greatly ex tended between its point of origin behind the endostyle and the basal zone where budding normally proceeds.
In the Pleurogona (e.g., Botryllus) these changes do not take place. The adhesive papillae are surrounded in the late larva by a marginal ring of additional processes (ampullae), fixation takes place over a much wider area, and the body normally retains a more or less horizontal attitude. The apex of the stolon is thus buried beneath the area of fixation, and only Molgulids preserve vestiges of the original epicardium. The parietal origin of the buds in this section is thus correlated with a distinctive type of metamorphosis, which is plainly secondary.
In Thaliacea the primitive ventral position of the epicardial stolon is retained, but the stolon itself presents a more complex structure, uniting the two modes which in Ascidiacea are sepa rated. It is traversed by a median or paired outgrowth from the back of the endostyle as well as by a pair of atrial diverticula. The buds are produced by direct segmentation of the stolon, and in Pyrosoma the various parental outgrowths give rise in the bud to the same organs as those from which they arise, i.e., the pharyngeal outgrowth' reproduces a pharynx, and the atrial out growths produce atria. In Salpa this specificity of the stolonial components is less complete, and in Doliolum it has largely dis appeared.