Fig. 15.
Perophora listeri; A, slightly, B, further magnified. Ascidiozooids in right, left, and lateral aspects.
a, branchial; b, atrial orifice.
The remarkable Rhopalæa neapolitana, from Naples, may be roughly compared to an hour-glass with a very long constriction. The test is smooth in the upper part, but knobby and encrusted with foreign bodies below. The upper or thoracic end contains the branchial sac, and the lower or abdominal portion the stomach, heart, and reproductive organs, the gullet and intestine traversing the whole length of the narrow central region. Although from its general structure Rhopalæa is a Clavelinid, it is not certainly known to produce buds.
Perophora listeri (Fig. 15) occurs in the form of little jelly-like transparent blobs rising by short stalks from a silvery thread-like stolon. Owing to their small size and transparency, it is possible to examine specimens alive under the microscope, the currents passing through the stigmata in the walls of the branchial sac, and the beating of the heart being distinctly visible. The rapid motion of the cilia surrounding stigmata gives the appearance of dark wheels all rotating in the same direction. The heart beats so as to drive the blood current so many times in one direction, and then after a short pause, in the reverse direction.
The exhibited specimen growing on an oyster shell, is from Plymouth.
Sub-order 2.—Ascidiæ Compositæ.
The Compound Ascidians are fixed forms, which give rise to colonies by budding, the individuals being immersed in a common mass and not possessing separate tests.
Although reduced to an extremely small size each individual or ascidiozooid of a colony possesses the same organs as a large Simple Ascidian, excepting that the former does not possess a separate test. Frequently the individuals of a colony are grouped into systems, in which the atrial orifices open into a common cloaca. The little ascidiozooids vary greatly in shape in the different families. In the Polyclinidæ, for instance, they are long, the organs being so to speak, drawn out, and being arranged in three regions, the thoracic, abdominal and post-abdominal, the first region containing the branchial sac, the second the stomach, and the third the heart and reproductive organs. In the Distomidæ, the body exhibits two regions, thoracic and abdominal, the heart and reproductive organs lying alongside of the stomach. The Botryllidæ comprise only one region, the stomach and the other organs being situated by the side of the branchial sac.
The Compound Ascidians include seven families which are characterised chiefly by the method of bud formation, and by the arrangement of the organs into one, two, or three regions.
It is only possible, from limits of space, to refer to a few interesting forms.