Page:Scientific results HMS Challenger vol 18 part 2.djvu/43

Rh striped (the expression of concentric lamellæ), and with spinulate surface, covered with innumerable small thorns. The basal quarter of each spine is straight and simple, the second quarter twice forked, and these four fork-branches are again in the outer half of the spine richly forked or ramified, with diverging, slightly curved thin branches; each of the twelve spines with about sixty to eighty terminal branches, the ends of which seem to fall into a spherical face. The position of this remarkable species in this family is doubtful.

Dimensions.—Length of the spines 0.12 to 0.16, of the simple basal part 0.04.

Habitat.—South Pacific (off Juan Fernandez), Station 299, surface.

5. Polyplagia viminaria, n. sp.

Numerous (sixteen to twenty or more) radial spines of about equal size, arising from a common central point and diverging in different directions, richly and more or less irregularly branched. The ends of the numerous small branches seem to fall into a spherical face. The large spines of this species have the same form and structure as in the preceding, nearly allied species, but are more numerous and more irregularly branched and disposed.

Dimensions.—Length of the spines 0.2 to 0.25, of the simple basal part 0.05.

Habitat.—North Pacific, Station 241, surface.

Plectanida, Haeckel, 1881, Prodromus, p. 424.

Definition.— with a wattled skeleton, composed of the meeting and united branches of radial spines, which arise from a common central point or central rod, and protect the partly enclosed central capsule.

The family comprises those  in which the skeleton is composed of radial spines, arising from a common centre, and of a loose wickerwork, produced by concrescence of the meeting branches of those spines. This rudimentary wattled skeleton is either quite irregular or only slightly regular, but it never assumes the form of a complete lattice-shell, as in the (the, , and ), nor does it exhibit a ring (as in the ). The central capsule is partly or wholly protected, and often entirely enclosed by the wattled skeleton.

Three species only of Plectanida have been hitherto known. The first described form is Plectophora arachnoides, which its discoverer Claparède observed in a living state in 1855 on the western coast of Norway, and considered as a mere variety of his Plagiacantha arachnoides. Two other species were afterwards observed in the Mediterranean, Polyplecta dumetum, 1856, by Johannes Müller (united by him with Acanthodesmia) and Polyplecta polybrocha by myself in 1864. Many new forms are found in the