Page:Scientific results HMS Challenger vol 18 part 2.djvu/32

908

Definition.— with three radial spines.

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

The genus Triplagia and the following closely allied Plagiacantha may be regarded as the simplest and most primitive forms of the, perhaps as the common ancestral stock of this suborder. The skeleton is composed of three simple or branched radial spines, arising from one common central point. These three spines in Triplagia lie in one and the same plane, whilst in Plagiacantha they lie in different planes. Therefore the former exhibits the simplest type of the triradial structure, common to the majority of.