Page:Scientific results HMS Challenger vol 18 part 2.djvu/45

Synopsis of the Genera of Plectanida.

I. Subfamily Triplectida. Three radial spines.	$\scriptstyle {\left\{{\begin{matrix}\ \\\ \end{matrix}}\right.}$	Three spines lying in one horizontal plane,			393. Triplecta.
		Three spines corresponding to the edges of a flat pyramid,			394. Plectophora.
II. Subfamily Tetraplectida Four radial spines.	$\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \\\ \\\ \ \end{matrix}}\right.}$	Four spines arising from one common central point.	$\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \ \end{matrix}}\right.}$	All four spines equal,	395. Tetraplecta.
				One apical spine opposed to three basal spines,	396. Plectaniscus.
		Four spines arising in two pairs from the poles of a common central rod; one apical spine different from three basal spines,			397. Periplecta.
III. Subfamily Hexaplectida. Six radial spines.	$\scriptstyle {\left\{{\begin{matrix}\ \\\ \end{matrix}}\right.}$	Six spines arising from one common central point,			398. Hexaplecta.
		Six spines arising in two opposite groups from the poles of a common control rod,			399. Plectanium.
IV. Subfamily Polyplectida. Numerous radial spines.		Numerous (seven to nine or more) radial spines arising from a common centre (either a central point or a branched rod),			400. Polyplecta.

Subfamily 1. Triplectida, Haeckel, 1881, Prodromus, p. 424.

Definition.—Plectanida with three radial spines.

Genus 393. Triplecta,^[1] Haeckel, 1881, Prodromus, p. 424.

Definition.—Plectanida with three radial spines, arising from one common central point and lying in one horizontal plane.

The genus Triplecta is the simplest and most primitive of the Plectanida, and may therefore be regarded as the prototype of this family. The skeleton represents a triangular lattice-plate with three radial beams. On the central union of the latter rests the oral pole of the central capsule. Since the axis of the latter is vertical, the lattice-plate must be horizontal, serving for the expansion of the pseudopodia. Triplecta has arisen from Triplagia by union of the branches of its three radial spines. In the simplest case only three large meshes are formed, corresponding probably to the three cortinar meshes in the collar septum of many Cyrtellaria.

1. Triplecta triangulum, n. sp.

Spines straight, equal, smooth, cylindrical, each in the basal half with one pair of divergent straight lateral branches. The opposed branches of every two neighbouring spines are united by a

1. Triplecta = Hunting net with three beams; τρίς, πλεκτή.