111. _The Skeleton of the Nassellaria._--The skeletons of the NASSELLARIA or MONOPYLEA consist of silica, and are never composed of separate portions, but constitute always a single continuous piece. The ground-form is originally monaxon, corresponding to that of the central capsule, with a constant difference between the two poles of the vertical main axis. The ground-form is never spherical or polyaxon as in the lattice-shells of the SPUMELLARIA, and the skeleton never consists of hollow tubes, as in the PHÆODARIA. The legion NASSELLARIA may be divided into two orders; in the #Plectellaria# (three suborders #Nassoidea#, #Plectoidea#, #Stephoidea#) the skeleton does not form a complete lattice-shell; in the #Cyrtellaria#, on the other hand, which are derived from these, the siliceous skeleton forms a complete lattice-shell enclosing the central capsule. The number of forms thus developed is astonishingly great, so that among the NASSELLARIA no less than two hundred and seventy-four genera and sixteen hundred and eighty-seven species may be distinguished, almost as many as in the #Sphærellaria#. In spite of this great variety of forms the legion MONOPYLEA is probably monophyletic; at least all the different skeletal forms may be derived from three elements which are combined in the most manifold fashion; (1) the _sagittal ring_, a simple siliceous ring, which lies vertically in the sagittal plane of the body, encircles the central capsule and comes into contact with it at the basal pole of the main axis (§ 124); (2) the _basal or oral tripod_, composed of three diverging radial spines, which meet in the middle of the basal pole of the central capsule (or in the centre of the porochora) (§ 125); (3) the _cephalis_, or lattice-head, a simple ovoid or subspherical lattice-shell, which encloses the central capsule and stands in connection with it at the basal pole of its main axis. Any one of these three important structural elements of the NASSELLARIAN skeleton may possibly be the starting-point {lxxvi}for all the remaining forms of the MONOPYLEA; the great difficulty in their phylogenetic derivation lies in the facts that, on the one hand, any one of the three elements may alone constitute the skeleton, and on the other hand, in the great majority of the legion, two or three are united together (compare §§ 182-185).