An otolith borne on a stalk is found in the same position in the larva. The Larvacea have a large vesicular otocyst containing an otolith on the left side of the anterior ganglion, and the 'nurse' in Doliolum has a similar structure on the left side of the body, connected by a nerve to the ganglion.
1 In certain simple Ascidians the ganglion is connected posteriorly to a nerve, 'the visceral ganglionic cord,' which contains ganglion cells, and passes along the dorsal edge of the pharynx, then between the rectum and so-called oesophagus, inclines to the right, and ends between the two lobes of the liver. It is derived from the main cord of the larva, which extends from the brain to the root of the tail and along that organ. See E. van Beneden and Julin, Archives de Biol. v. 1884, PP. 317-321, and cap. II. p. 337 et seqq.
The oral or 'inhalent' aperture leads into a pharynx. A groove - the endostyle - extends along the ventral aspect of this cavity, except in Kowalewskaia, which has in its place a right and left row of ciliated tooth-like processes. This endostyle is lined by mucus-producing cells and ciliated cells. Anteriorly its lips are connected with the posterior of two ciliated peripharyngeal bands surrounding the oral aperture, which is continuous on the dorsal aspect of the pharynx with a ciliated ridge or fold, the dorsal lamina. This lamina extends backwards to the opening of the 'oesophagus.' Its edge is sometimes denticulated, forming a single or double row of languettes. Posteriorly the endostyle is connected to the oesophagus by a ciliated line, which comes also into connection with the posterior end of the dorsal lamina. The mucus secreted is chiefly carried forwards, and as it passes upwards to the dorsal lamina, it is drawn out into long strings by the current of water entering the oral aperture, and catches up all floating food-particles with which it is laden. It is then conveyed along the dorsal lamina into the oesophagus.
In Larvacea the pharynx has two latero-ventral outlets or stigmata, one on the right, the other on the left, surrounded by a ring of ciliated cells, and developed each from a pharyngeal outgrowth meeting an ectodermal ingrowth. Other Urochorda have a single atrial, exhalent or cloacal aperture. This aperture is placed dorsally, either near the oral aperture (Ascidiae Simplices and some Compositae) or at some distance from it (other Compositae), or else at the opposite pole of the body (A. Salpaeformes, Thaliacea). It leads into a 'peripharyngeal' or atrial cavity, which surrounds the pharynx, except along the line of the endostyle, and is limited anteriorly by the line of the peripharyngeal bands. The cavity is lined by an epithelium derived from the ectoderm, from which it is formed by one or two invaginations. In the latter case the two apertures fuse. In certain buds, however, it appears to be formed by pharyngeal outgrowths. The lateral walls of the pharynx are often folded longitudinally and are perforated by ciliated slits (stigmata), between which run blood-channels. The slits lead into the atrial cavity.
Among Thaliacea, the pharynx in Doliolum is represented by a vertical membrane perforated by only two rows of slits, and in Salpa it is reduced to the dorsal lamina with a great gap on either side of it.
The digestive portion of the alimentary canal is formed as an outgrowth of the archenteron. It is ciliated in part or wholly, especially the intestinal section. It lies either behind the pharynx, or to the left side of it (some A. Simplices and Compositae), or is reduced in size and placed quite to the ventral side, as in Pyrosoma, Doliolum, and Salpa, and in the last it forms, with the other viscera, a small mass termed 'nucleus.' The first portion of it ('oesophagus') is narrow, and variable in length. The stomachal region is more or less dilated, and the intestinal tubular. The anus opens on the ventral surface between the stigmata, or on the right side in Larvacea. In other Urochorda the intestine has an adoral and dorsal twist, and the anus opens just within the atrial aperture, i.e. is dorsal in position. When this aperture is terminal and posterior, the course of the intestine is still towards the dorsal aspect. In Larvacea there are no specialised gland cells at all in the digestive tract. Glandular caeca coat the stomach in many A. Simplices and Compositae, and in the Molgulidae form a lobed mass.
In most Urochorda a system of clear tubes ramifies over the intestine and sometimes the stomach as well, occasionally anastomosing freely (some Salpae) and ending in ampullae (Ascidia, Perophora Salpa, Pyrosoma). They open by one or two large tubes into the posterior part of the stomach. Their function is not clear, but cilia have been detected in them in Perophora and Doliolum.
A heart is absent only in Kowalewskaia. In other Larvacea it lies more or less ventrally close to the stomach, is elongated transversely, and at either end of the longer axis is a large cell. Between the two cells stretch muscle fibres. In other Urochorda the heart is tubular, with circular muscle cells, and is contained in a pericardial sheath. It lies ventrally on the right or left side, close to the stomach, in front of it, along it, or, in Clavellinidae and many A. Compositae, behind it. In all Urochorda the direction of its beats undergoes periodic reversal. In Larvacea there are no vessels and no blood corpuscles - nothing but irregular and small spaces representing a coelome. The direction of the currents can only be detected when the blood is infested with parasitic organisms. Other Urochorda possess vessels in connection with both ends of the heart. A main blood-channel lies below the endostyle, another above the dorsal lamina. The two are connected by transverse vessels encircling the pharynx, which in their turn are connected by minute longitudinal interstigmatic vessels.
The latter are absent in Pyrosoma. There are sometimes large longitudinal vessels running on the inner surface of the pharynx, connected to the transverse vessels where they cross them, e.g. in Pyrosoma, many Ascidiae, etc. Branches from the pharynx pass into the body walls and test. The visceral vessels are connected to the dorsal vessel of the heart and pharynx.