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



and possesses neither skeleton nor alveoles, nor other differentiated parts. The innumer- 

 able 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 phylogeneticaUy from the simplest 

 skeletonless Heliozoa [Actino2'>hrys, Heterophrys, ActinosphcBrium, SphcBvastrum). 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 as 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. 



16L Hypothetical Genealogical Tree of the Spumellaria (see opposite page). 



162. Colloclaria and Sphmrellaria. — -Whilst in all Spumellaria the malacoma agrees 

 in possessing the characteristic features of the legion, and thus justifies its derivation mono- 

 phyletically from the common stem-form ^c^ma, the different forms of skeleton, on the other 

 hand, cannot aU be referred to the same fundamental form. More especially the spherical 

 lattice-shell, from which all the numerous skeletal forms of the Sphserellaria may be 

 derived, cannot have arisen from the incomplete Beloid skeleton which characterises the 

 Beloidea among the C o 1 1 o d a r i a. It is probable rather that the formation of the 

 skeleton has taken place independently in those two groups of Spumellaria. From the 

 skeletonless C o 1 1 o i d e a, as the common stem-group of the Spumellaria, two different 

 main groups have diverged, on the one hand the Beloidea, whose skeleton consists of 

 separate spicules scattered in the extracapsulum, and on the other hand, the S p h se r e 1- 

 1 a r i a, which have formed a simple lattice-sphere around the central capsule ; from this 

 the manifold forms of the remaining Spumellaria may be derived. 



163. Descent oj the SphcBvellaria.- — -The extensive order Sphserellaria, which 

 includes aU Spumellaria 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, Ceiwsphcera. The main 

 stem of the order, the extensive suborder Sphseroidea (Pis. 5-30), is derived 

 immediately from CenosphcBra (p. 61, PL 12); three diverging branches of it being 

 represented by the other three suborders, the Prunoidea (Pis. 16, 17, 39, 40) 

 being developed by elongation, and the Discoidea (Pis. 31^8) by shortening of 

 the vertical main axis, whilst the Larcoidea (Pis. 9, 10, 49, 50) have originated 

 by the modification of the spherical lattice-shell into a lentelliptical or triaxial eUijjsoidal 

 one. Although the raonophyletic derivation of all S p h se r e 1 1 a r i a from CenosphcBra is 

 exceedingly probable, the possibility of a polyphyletic origin for the group is by no 



