Page:Scientific results HMS Challenger vol 18 part 1.djvu/138

cxviii 181. Plectellaria and Cyrtellaria.—The extensive legion far surpasses the other three legions in the endless variety of its skeletal structures, and owing to the complicated relationships of its numerous families presents no lack of difficult phylogenetic problems. All may be divided first into two main groups or sublegions,  and ; the latter having a complete lattice-shell, the former not. Probably the have been polyphyletically developed from several different groups of. These groups are, however, connected in such manifold ways that a monophyletic origin of all the skeletons from one original element is possible. Such a primitive element may have been furnished by any one of three different skeletal parts, the sagittal ring, the basal tripod, and the latticed cephalis (compare pp. 891-895, Bütschli, L. N. 40, 41).

182. Phylogenetic Skeletal Elements of the Nassellaria.—The multiform skeleton of the may be referred in different ways to one of the three above-mentioned structural elements. Each of these (p. 891) may by itself form the skeleton; the sagittal ring in the simplest (Archicircus, Lithocircus), the basal tripod in the simplest  (Triplagia, Plagiacantha), the latticed cephalis in the simplest  (Cyrtocalpis, Archicapsa). In the great majority of the, however, two of these elements, or even all three, are found combined. In most, more especially, both the sagittal ring and the basal tripod may be recognised in the lattice-shell, though often only in slight rudiments or scarcely perceptible traces. In the also (which possess no latticed cephalis) there are individual genera with complete development both of the sagittal ring and basal tripod; this important combination is especially well represented in the Cortinida (Cortina, Cortiniscus, Stephanium, Stephaniscus, Tripocoronis, &c.). The greatest difficulty as regards the phylogeny of the lies in the fact that the most various combinations of the three elements are presented by closely related or very similar forms. If, in spite of this, a monophyletic hypothesis as to the origin of the seems essential all sides of the three possible hypotheses must receive full consideration and critical comparison (§§ 183-191).

183. Ascent of the Nassellaria from the Plectoidea.—The monophyletic hypothesis (No. 2, p. 893) which regards the basal tripod as the common origin of the skeleton of all, starts from the simplest forms of the (Triplagia, Plagoniscus, Triplecta, Plectaniscus, &c., Pl. 91). All may be immediately derived as diverging twigs of these, as well as all triradial and multiradial forms of  and ; for in all these cases the distinctive triradial (or the derived multiradial) form of skeleton appears directly derivable from the simple basal tripod of the former. The same is perhaps also true of many.