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



axes on the one hand ( 44, 45), and the differentiation of the various parts of the 

 unicellular organism with the formation of the skeleton on the other, are of pre-eminent 

 significance. Even in the varying growth of the central capsule in the different 

 dimensions of space in the skeletonless Colloidea, four different modes may be 

 distinguished, which further, in the corresponding development of the skeleton, furnish 

 the basis for the origin of the four orders of Sphaerellaria. The most primitive 

 and simplest form of growth, equal extension in all directions, is found in the spherical 

 central capsule and the concentric spherical skeletons (Procyttarium, SphaBroidea). 

 When the growth of the central capsule proceeds more rapidly in the direction of the 

 vertical main axis than in any other direction, the ellipsoidal or cylindrical central capsule 

 (Actiprununi) arises, and the vertically elongated skeleton of the Prunoidea, which 

 is derived from it. When, on the contrary, the growth of the central capsule and lattice- 

 shell is less in the direction of the vertical main axis than in any other direction, the 

 lenticular or discoid central capsule (Actidlscus) arises, and the corresponding lenticular 

 shell of the Discoidea. Finally, even quite early in many SPUMELLARIA, the growth 

 of the central capsule and of the corresponding lattice-shell in the three dimensive 

 axes is different, and hence arise the lentelliptical forms whose geometrical type is the 

 triaxial ellipsoid or the rhombic octahedron (Actilarcus, Larcoidea). Thus the 

 origin of the four orders ofSphserellaria is simply explained by a varying growth 

 in the different dimensive axes. The primary (innermost) lattice-shell is in this legion 

 always simultaneously developed (suddenly excreted at the moment of lorication from 

 the sarcodictyum). The secondary lattice-shells, on the other hand, which surround the 

 former concentrically, and are united with it by radial bars, arise successively from within 

 outwards. 



147. The Ontogeny of the Acantharia. The individual development of the 

 ACANTHARIA in the simplest case (Actinelius) stops at a point which differs from the 

 Actissa-st&ge only in the change of radial axial threads into acanthin spines. Jn the 

 small group Actinelida, their number remains variable and usually indeterminate 

 (Adelacantha), whilst in the great majority of the legion (Acanthonida and A c a n- 

 thophracta) the number is constantly twenty, and those spines are regularly arranged 

 according to the Mlillerian law in five parallel circles, each containing four crossed spines 

 (Icosacantha). The simplest form among these latter is Acanthometron, which may be 

 regarded both ontogenetically and phylogenetically as the common starting-point of all 

 the Icosacantha. Within this extensive group variations in the length of the dimensive 

 axes appear, similar to those observed in the SPUMELLARIA. In the Astrolonchida and 

 Sphserophracta the central capsule remains spherical, extending equally in all 

 directions ; and correspondingly the lattice-shell, which is excreted on the surface of the 

 spherical calymma, remains spherical. In the Belonaspida (just as in the Prunoidea) 



