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



181. Plectellaria and Cyrtellaria. — The extensive legion Nassellaeia 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 phylo- 

 genetic problems. All Nassellaeia may be divided first into two main groups or 

 sublegions, Plectellaria and Cyrtellaria; the latter having a complete lattice- 

 shell, the former not. Probably the Cyrtellaria have been polyphyletically developed 

 from several different groups of P 1 e c t e 1 1 a r i a. These groups are, however, connected 

 in such manifold ways that a monophyletic origin of all the Nassellaeian 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 (comj^are pp. 891-895, and Biitsehli, L. N. 40, 41). 



182. Phylogenetlc Skeletal Elements of the Nassellaria. — The multiform skeleton of 

 the Nassellaeia may be referred in different ways to one of the three above-mention e,d 

 structural elements. Each of these (p. 891) may by itself form the skeleton; the 

 sagittal ring in the simplest S t e p h o i d e a [Archicircus, Lithocircus), the basal tripod 

 in the simplest Plectoidea {Tri'plagia, Plagiaxantha), the latticed cephalis in the 

 simplest Cyrtoidea {Cyrtocalpis, Arehicajjsa). In the great majority of the Nassel- 

 laeia, however, two of these elements, or even all three, are found combined. In most 

 Cyrtellaria, 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 Plectellaria 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 Nassellaeia lies in the fact that the most various com- 

 binations 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 Nassellaeia 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 hyjjothesis 

 (No. 2, p. 893) which regards the basal tripod as the common origin of the skeleton of 

 all Nassellaeia, starts from the simplest forms of the Plectoidea {Triplagia, 

 Plagoniscus, Triplecta, Plectaniscus, kc.,V\. 91). All Plectoidea may be imme- 

 diately derived as diverging twigs of these, as well as all triradial and multiradial forms 

 of C y r 1 i d e a and S p y r o i d e a ; 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 B o t r y o d e a. Further- 



