Tergum

Definition and stakes

Portrait of Charles Darwin Charles Darwin,  A Monograph on the Sub-class Cirripedia…

“ The Tergum, in rather small specimens is, if the upper imbedded portion be included, sub-triangular, with the growing surface oval, and between one third and one half of the length of the scutum; but in large specimens, the tergum becomes style-formed, lying parallel to the tergal margin of the scutum, with the growing surface proportionally much smaller, and not above one fifth of the length of the growing surface of the scutum. Hence we see some tendency in the tergum to become rudimentary, as it is in the next species. ”
Source: Gutenberg

Portrait of Charles Darwin Charles Darwin,  A Monograph on the Sub-class Cirripedia…

“ The colour also varies; specimens of T. porosa (Pl. 10, fig. 1 a to 1 f) being dark purple, or even inky black, white, pale pinkish-purple, and green: as far as I have seen, the pinkish varieties of T. porosa are confined to the eastern half of the world. Even the ridges and crests on the under side of the scutum vary in some degree; and the tergum in T. porosa varies considerably, and in some of the other species, slightly, in general shape: I believe that the tergum becomes narrow and elongated, when the shell is steeply conical, with the orifice of small diameter. ”
Source: Gutenberg

Portrait of Charles Darwin Charles Darwin,  A Monograph on the Sub-class Cirripedia…

“ The Terga differ considerably in outline in the different genera and species: their shape approaches more nearly to a triangle than to any other regular form; but there is generally a projection or spur on the basal margin, on the side towards the scutum. In some species of Pyrgoma, the tergum is of so irregular a shape as to defy description. In most cases, a longitudinal depression or furrow runs down the valve, from the apex to the extremity of the spur; and it not rarely happens that the sides of this furrow become folded inwards and almost closed. ”
Source: Gutenberg

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