. The British Tunicata; an unfinished monograph. Sea squirts; Tunicata. 99 BKIT1SH TUNTCATA. The chief function of the test, like that of the shell in the higher mollusks, is no doubt to protect the comparatively soft and delicate portions of the animal that lie within it. But it will also act, by its resiliency, as a counterpoise to the muscular contractility of the mantle, which lines it as it were. In those species, such as Styela tulerosa, in which the mantle and test are adherent throughout, this action is readily understood ; it is not,, however, quite so obvious in the species which hav
. The British Tunicata; an unfinished monograph. Sea squirts; Tunicata. 99 BKIT1SH TUNTCATA. The chief function of the test, like that of the shell in the higher mollusks, is no doubt to protect the comparatively soft and delicate portions of the animal that lie within it. But it will also act, by its resiliency, as a counterpoise to the muscular contractility of the mantle, which lines it as it were. In those species, such as Styela tulerosa, in which the mantle and test are adherent throughout, this action is readily understood ; it is not,, however, quite so obvious in the species which have these two tunics comparatively free, as they are universally in Ascidia and Molgula. But we have just seen that, in such Distal or anterior. Dorsal. Proximal or posterior. FIG. 1.—Ascidia scabra. Left side. t. Test (outer tunic). m. Mantle (inner tunic), Branchial aperture of test. Branchial aper- ture of mantle, Atrial aperture of test. At rial aperture of mantle. Three times natural size. From a specimen preserved in spirit, with the mantle shrunk. instances, the inner surface of the test, and the outer surface of the mantle, lie in close contact with each other. Now, as under all ordinary circumstances the pressure of the water inside the mantle must be as great as that of the water rest- ing against the outer surface of the test, and as no water can possibly enter between these two tunics, it is clear enough that they will be held together with no inconsiderable force. Thus, when the muscles of the mantle contract, diminishing the bulk of that organ, the test will be drawn in after it; and so soon as the muscles of the former relax, the latter, through the elasticity of its walls, will expand, and the mantle will be constrained to do so Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations
Size: 1294px × 1932px
Photo credit: © Library Book Collection / Alamy / Afripics
License: Licensed
Model Released: No
Keywords: ., bookcentury1900, bookdecade1900, bookpubli, booksubjectseasquirts
