ANATOMY OF SALM. - The subcartilagi nous test of Salpw is more or less cylindrical and flexible, and when taken out of the sea, often collapses into an amorphous mass. Float ing in the water, it is iridescent or opaline, reflecting the sun's rays in beautiful rainbow tints, and hence has been derived its name of " sun-fish." The test and its lining membrane are so diaphanous, that the whole structure of the animal can be seen through them. The exterior of the test is generally smooth, but sometimes bears minute shiny protuberances of a tissue similar to its own, as in S.nlesii. The integument over the viscera is thicker and often of a firmer consistence than the rest of the test. Its form varies considerably, not only in different species, but in the conditions of isolation and aggregation, in which each species alternately exists, and also in the different stages of growth of the individual, especially in an associated state.
The test is usually either oval or oblong, but various terminal and lateral processes considerably affect its general contour. Each of its extremities is open by an orifice ; and, in the aggregated individuals, it is perfo rated by other much smaller apertures at the points of contact between the neighbour ing individuals of a group.
The internal surface of the test is lined by the mantle, a thin, toughish membrane, which is often more conspicuous than the highly transparent external envelope. The mantle is furnished with large, generally transverse, muscular bands, the arrangement of which differs according to the species and according to the separate or associated state of existence. Its inner surface is lined with a soft mucous coat of fine epithelial tissue. The mantle is more or less closely attached to the inner sur face of the test, especially at the two orifices (being, indeed, continuous with the test at these points), at the protuberances that arise from the test, and along the inner surfaces of the longitudinal furrows that sometimes tra verse its external surface, as in S. cordiformis, where, on the superior and inferior surfaces, the test is depressed into broad, deep sulci.
In specimens preserved in spirit, and even in some recent specimens, this membrane will sometimes separate itself, together with the viscera and vessels, from the outer tunic, and fall through one of the external orifices. Salpfe have been met with floating in the sea, and executing, in some degree, their usual movements of contraction and dilatation; in which, from mutilations caused by parasites or other accidents, little remained except a few muscular bands of the mantle.
Considerable confusion has existed among naturalists as to which is the anterior extre mity, and which the superior surface of the Salpa. We shall regard, as the anterior or branchial orifice, that in the neighbourhood of which the stomach and heart are usually found (fig. 787.); and as the posterior or anal orifice, that which points in the direction of the ani mal's retrograde motion ; and as the superior or dorsal surface, that on which the nerve ganglion is present; and as the inferior or ventral, that in close contact with which the viscera and heart, forming the " nucleus " or " paquet" of authors, are placed.
The orifices are either terminal, or are situ ated at the base of the terminal prolongations of the test. The anterior orifice is destitute of tentacular appendages ; it is furnished with sphincter muscles ; it is simple and tubular in S. cristata, S. Taegu, and S. seutigera, &c., and transverse in S. S. zonaria, &c. When the dilatation of the animal is drawing water into the cavity of the body, the sphincters of this orifice are brought into action and close it, so that the water enters by the opposite extremity.
The posterior orifice is a transverse slit, furnished with a few muscular fascicles, and is larger than the opposite orifice. In S. eris tata its superior lip is simple and thin ; its inferior lip is externally a rounded ridge, formed by a fold of the test, which, within its inner border, constitutes a semilunar valve. This allows the water to enter easily, but pre vents its escape when the animal contracts itself.