ANATOMY AND PHYSIOLOGY OF THE CATA.
It will be most convenient to describe the test generally for the whole tunicate class, and the rest of their anatomy under the heading of each of the families.
Test or Shell. — The test or external enve lope of the Ascidiada is subject to considerable variation of shape ; from the bottle-like form of the Ascidia (fig. 766.), to the flat, patella like form of Chelyosonza (fig. 767.); it is elastic, varying very much in its thickness and con sistence in different species. In colour it varies considerably, being occasionally nearly black, sometimes red, orange, yellow, or milky white. Its surface is sometimes smooth, often tuberculated, covered with hairs or spines, or otherwise roughened. Sometimes minute patches of horny tissue or hardened epidermis rise up irregularly on the surface of the test ; in other cases these are placed in a tessellated arrangement. In the peculiar and unique form, Chelyosonza, first noticed by Messrs. Broderip and Sowerby, and since more fully described by Professor Eschricht, the upper surface of the test is occupied by eight large, horny, polygonal plates arranged some what like the shell-plates of a tortoise, and by several smaller triangular plates, which form two circles, one around the branchial, and the other round the anal aperture of the animal (fig. 767.). The large plates are so disposed that the branchial orifice is surrounded by three plates, and the anal by four, besides that which is intermediate and abuts upon both : this latter plate is hexagonal, the sides in con tact with the orificial valvules are lunated. The three plates near the branchial orifice are much larger than the four which are near to the anal orifice. Each of the plates is marked with three or four elevated striae, which are near to the edges of the plate, and parallel with them, leaving an area in the centre, and giving rise to a general resemblance to the external plates of the shell of a land tortoise. The orifices are very small, and are surrounded by six triangular valvules, each transversely stri ated, and, when shut, rising from the sur rounding surface in the form of a cone. The lower or adherent part of the test of Chely °soma is coriaceous, with occasional slight traces of separation into plates.
The test of Ascidice is frequently covered with innumerable smaller animals and their spawn. Modiolm and Annelids burrow in it ; Cirrhipeds, naked Molluscs, and Actinim lodge upon it ; and Corallines cover it sometimes with a little forest ; a condition fully justify ing the denomination of " microcosmus," be stowed by Redi # on a Mediterranean species. Occasionally, as in Ascidia conchilega and Mol gula oculata, the animal works up extraneous matter, as gravel, fragmentary shells, &c., with the external surface of the test. It is by this shell or test that the animal fixes itself. In the sessile species, the tissue of the base or the side of the test interlaces with the stems of sea weeds and corallines, or closely adheres to the surface of another ascidian sac, or of a stone, a shell, a crab, or other object. In the peduncled forms this tunic is at one point pro longed into a tubular process or stem, the dis tal extremity of which is attached to marine bodies in the same manner as the base of the sessile tests. In Cystingia and Bipapillwria, however, this process or stalk appears to be less perfectly developed, and not to be al together adapted to maintain a permanent attachment to a fixed body.
The external envelope of the Ascidiada. is always perforated by the two apertures characteristic of the Tunicata, and analogous to the prolonged respiratory orifices of the Cardiacece and other Acephala. One of these apertures*, nearly always placed at the sum mit of the test, receives the sea-water, and admits it into the branchial cavity. The second aperture t is placed a little lower than the first, and is in communication with the rectum and oviduct. In Boltenia the ori fices are lateral ; in Cystingia the branchial orifice is lateral, and the anal terminal ; the oral orifice being always the highest in relative position, and nearest to the insertion of the pedicle by which the animal is suspended. In C'hclyosonza and Dendrodoa the orifices are placed on the same plane ; in the former, on the nearly flat superior surfiice of the animal, and in the latter they are terminal.