Page:Scientific results HMS Challenger vol 18 part 1.djvu/126

cvi and possesses neither skeleton nor alveoles, nor other differentiated parts. The innumerable fine pseudopodia, which issue from the central capsule through the evenly distributed pores in its membrane, radiate in all directions through the calymma and pass out over its surface. Actissa can, therefore, be directly derived phylogenetically from the simplest skeletonless Heliozoa (Actinophrys, Heterophrys, Actinosphærium, Sphærastrum). The only essential difference between the two consists in the development of the central capsule, which in Actissa separates as a distinct membrane the endoplasm from the exoplasm. This differentiation which we regard is the most important distinguishing character of the Radiolaria, has been transmitted by inheritance, along with the formation of flagellate spores in the central capsule, from Actissa, the primitive parent to all the other Radiolaria.

161. Hypothetical Genealogical Tree of the Spumellaria (see opposite page).

162. Collodaria and Sphærellaria.—Whilst in all the malacoma agrees in possessing the characteristic features of the legion, and thus justifies its derivation monophyletically from the common stem-form Actissa, the different forms of skeleton, on the other hand, cannot all be referred to the same fundamental form. More especially the spherical lattice-shell, from which all the numerous skeletal forms of the may be derived, cannot have arisen from the incomplete Beloid skeleton which characterises the  among the. It is probable rather that the formation of the skeleton has taken place independently in those two groups of. From the skeletonless, as the common stem-group of the , two different main groups have diverged, on the one hand the , whose skeleton consists of separate spicules scattered in the extracapsulum, and on the other hand, the , which have formed a simple lattice-sphere around the central capsule; from this the manifold forms of the remaining may be derived.

163. Descent of the Sphærellaria.—The extensive order, which includes all with a complete lattice-shell, develops an extraordinary variety of skeletal structures; these may, nevertheless, all be derived without violence from a common stem-form, or simple spherical lattice-shell, Cenosphæra. The main stem of the order, the extensive suborder (Pls. 5-30), is derived immediately from Cenosphæra (p. 61, Pl. 12); three diverging branches of it being represented by the other three suborders, the  (Pls. 16, 17, 39, 40) being developed by elongation, and the  (Pls. 31-48) by shortening of the vertical main axis, whilst the  (Pls. 9, 10, 49, 50) have originated by the modification of the spherical lattice-shell into a lentelliptical or triaxial ellipsoidal one. Although the monophyletic derivation of all from Cenosphæra is exceedingly probable, the possibility of a polyphyletic origin for the group is by no