900 THE VOYAGE OF H.M.S. CHALLENGER. 



the other families of this legion. But a very difficult and as yet unsolved problem is 

 the important question, in what manner these different groups of NASSELLARIA are 

 phylogenetically connected. Either the Plectoide a as the simplest of all are 

 the original common ancestral group of this whole legion (as I assumed in my 

 Prodromus, 1881), or they are derived from the Stephoidea (by reduction of the 

 sagittal ring), or they have originated independently from them (if we suppose a poly- 

 phyletic origin of the MONOPYLEA. Compare above, p. 893, &c.). In any case the 

 typical " triradial structure " of the Plectoide a, prevalent also in the other groups of 

 this legion, is a very important and interesting fact. 



The triradial skeleton of the Plectoidea exhibits in the two families of 

 Plagonida and Plectanida a complete homology of development, so that each genus of 

 the latter may be derived from a corresponding genus of the former, simply arisen by 

 concrescence or union of the branches of the radial spines. Therefore the only difference 

 between the two closely allied families is, that the branches of the radial spines in the 

 Plagonida remain free, whilst in the Plectanida they produce a loose framework or 

 wickerwork by union of their meeting ends. We express this complete homology in 

 the nomenclature of the Plectoidea, in each genus of Plagonida retaining the 

 syllable "Plag-" ; in each genus of Plectanida, correspondingly, the syllable "Plect-." 



The number of radial spines composing the skeleton is originally three, and in all 

 not triradial genera is probably derived from three. For better survey we may divide 

 each family, according to the different number of rays, into four different subfamilies : 

 A, with three radial spines (Triplagida and Triplectida) ; B, with four radial spines 

 (Tetraplagida and Tetraplectida) ; C, with six radial spines (Hexaplagida and 

 Hexaplectida) ; and D, with numerous (seven to nine or more) radial spines (Polyplagida 

 and Polyplectida). The last three subfamilies have arisen probably from the first 

 triradial subfamily, by a secondary increase in the number of rays. 



The important signification of the triradial structure, recurring in the most different 

 groups of NASSELLARIA, has been already pointed out sufficiently by myself and by 

 R Hertwig. But the triradial Plectoidea offer also another interesting relation of 

 this characteristic structure, some simple forms of this order appearing nearly identical 

 with the isolated triradial spicula of certain Beloidea (Thalassosphserida and 

 Sphserozoida). Even some more complex quadriradial and sexradial forms of the 

 latter reappear in exactly the same shape also in the former. This identity may be 

 perhaps an important indication of true affinity (compare below). 



The simplest and probably the most original kind of triradial structure is exhibited 

 by the genera Triplagia and Triplecta (PI. 91, figs. 2, 7). Here three equal radial 

 spines lie in one horizontal plane and are united in a common central point at equal 

 angles, so that three lines connecting their distal ends form a regular equilateral 

 triangle. Simple triradial spicula of the same regular form are also found in many 



