Dissected to show the chief features in the anatomy of Urochorda.
THE animal is mounted with the base of attachment or posterior end downwards, and the oral or inhalent aperture or anterior end upwards. The ventral edge is on the left hand, the dorsal on the right, and the animal's left side is therefore turned to the observer. On the dorsal edge, and somewhat anteriorly, is situated the atrial or exhalent aperture, through which a white bristle has been passed.
There is an external transparent test. This test is secreted by the ectoderm cells, and is remarkable among animal tissues for containing cellulose or a substance very closely akin to it. It is prolonged inwards for a short distance at both oral and atrial apertures. The body walls, often termed mantle, have shrunk away from the test under the action of the alcohol in which the animal is preserved. This contraction does not take place in all species of the genus, and in this instance the body walls remain firmly united to the test in the region of the two apertures. They may be seen to be fibrillated, and consist of connective tissue with bundles of muscular fibres. In the genus Ascidia these bundles are more or less irregularly arranged, and interlace more or less with one another, a fact to be made out easily in the part of the body walls left near the atrial aperture.
But in the genus Cynthia they are disposed in two layers, an outer longitudinal and an inner circular layer.
The cavities of the pharynx and digestive tract have been displayed by the removal of their own and the body walls on the left side. Close to the margin of the oral aperture may be noticed, first, the circle of tentacles which guard the entrance, secondly, behind this circle two delicate bands, the peripharyngeal bands which run circularly round the pharynx, and thirdly, the cavity of the pharynx itself. This cavity is marked by transverse and longitudinal lines at right angles to one another, inclosing innumerable square meshes. The transverse lines are the transverse vessels which run from the ventral to the dorsal edge of the pharynx, connecting the two main longitudinal bloodvessels which correspond to these two edges respectively. The longitudinal lines are the longitudinal bloodvessels which run on the inner surface of the pharyngeal cavity. They extend from the anterior to the posterior end of the pharynx, and are connected by short vessels to the transverse vessels at the spot where the two sets Cross one another. The longitudinal vessels which pass from one transverse vessel to another, and lie in the same plane as they do, are very fine and inclose the stigmata or apertures which lead from the cavity of the pharynx to the peribranchial or atrial cavity.
They are not visible without the use of the microscope. The cilia which beset the stigmata cause a current of water to flow from the pharyngeal into the atrial cavity. The pharynx has thus a respiratory function and is hence often termed branchial sac.
The pharynx extends on the right side of the animal to its posterior extremity, and the digestive portion of the alimentary canal is placed to its left side, an arrangement which does not occur in all Urochorda, e.g. Cla-velinidae. Stretching along the dorsal edge of the pharyngeal cavity is a delicate longitudinal band, the dorsal lamina. Anteriorly it may be seen to come into contact with the peripharyngeal bands. At this point there is an opaque white spot, the nerve-ganglion which is underlaid by the neural gland, and in front of the opaque spot may be noted a crescentic curved line with the concavity turned forwards. This line represents the dorsal tubercle, so-called, or the aperture of the neural gland. When the specimen is held in a certain position as regards the light, a yellow opaque line may be seen running along the ventral edge of the pharynx internally. This line is the endostyle, which extends from the peripharyngeal bands anteriorly to the posterior end of the dorsal lamina, close to which is the aperture from the pharynx into the stomach.
A black bristle has been passed down the pharynx and through this aperture.
The pharynx is attached to the body walls along the line of the endostyle, but a cavity, the atrial or peribranchial cavity, which opens externally by the atrial aperture, extends round its dorsal edge throughout nearly its whole extent to the right and left sides, separating them from the body walls. This atrial cavity is produced by two invaginations from the exterior in the larva, the openings of which unite, forming the single atrial aperture. The atrial must not be confounded with the body cavity which, if present at all, is probably represented by blood spaces in the body walls, etc.
The stomach is posterior and ventral: the intestine rises from its anterior ventral end, runs forwards for a short space, and then turns back upon the stomach dorsally and posteriorly. It passes to the left side of the aperture of the pharynx into the stomach, and then turns forwards and dorsally to end in the anus, which may be seen just above the dorsal edge of the pharynx internally to the atrial aperture. The cavity of the intestine is partially divided by a ridge or typhlosole which projects from its right wall, and commencing at the pyloric end of the stomach, extends in this Ascidian to within a short distance of the anus. In the substance of the body walls, which are left along the ventral edge of the intestine close to its origin, and in the concavity of its last curve may be noticed some opaque white cords. These are probably portions of the testis.
The substance of the test is sometimes fibrillated: it contains cellular elements which wander into it from the ectoderm. These cells increase the quantity of matrix. Some of them may become pigmented and others frequently undergo extreme vacuolation, becoming little more than bladders, with a thin wall of protoplasm which contains the nucleus. Chemically, the matrix appears to be identical with the cellulose of plants.