Page:Scientific results HMS Challenger vol 18 part 1.djvu/840

632 inclination to individual abnormalities, formation of varieties and transitions into other families, hence derived, as Larnacida, Tholonida, Lithelida, &c.

The central capsule in all Pylonida, in which I could observe it, was a true lentellipsis (or a "triaxial ellipsoid" in the geometrical sense, with the three unequal isopolar axes of the "rhombic octahedron"). In the living Pylonida it is commonly coloured pink or scarlet. During growth its dimensions are probably more or less changed, and perhaps the axes alternate. Regarding the relation of the central capsule to the skeleton, we can distinguish two different groups, quite as in the nearly allied Larnacida. In the Haplozonaria (as also in the Larnacillida) the central capsule encloses only the central chamber and is enveloped by the first system of girdles, whereas in the Diplozonaria and Triplozonaria (as in the Larnacalpida) that "trizonal shell" becomes enclosed (as the "medullary shell") in the central capsule, which is now enveloped by the second system of girdles as the "cortical shell."

Definition.— with one single system of fenestrated girdles (with one, two, or three girdles, lying in one lentelliptical face).