Ixxviii THE VOYAGE OF H.M.S. CHALLENGES. 



116. The Sphceroid Skeletons or Lattice-Spheres. The " lattice-spheres " or sphseroid 

 skeletons are the simplest and most primitive forms of lattice-shells, and are widely dis- 

 tributed in the three legions SPUMELLARIA, ACANTHARIA, and PH^ODARIA, whilst they 

 are entirely wanting in the NASSELLARIA. The round lattice-shell is either a true sphere 

 in the geometrical sense, or an endospherical polyhedron, i.e., a polyhedron, all whose 

 angles lie in the surface of a sphere ( 25). In general, primary and secondary lattice- 

 spheres may be distinguished, of whicli the former are secreted on the outer surface of 

 the primary, the latter on that of the secondary calymma ( 85). Furthermore, simple 

 and compound lattice-spheres may be distinguished, the latter of which consist of two 

 or more concentric lattice-spheres firmly united by radial bars ; in such cases the inner- 

 most lattice-sphere is always to be regarded as the oldest or primary, all the succeeding 

 ones as secondary, and the outermost as the youngest ( 129). The simple lattice-spheres 

 are usually to be regarded as primary; they may, however, occasionally be secondary, in 

 which case the primary shell, originally enclosed, has been lost by degeneration (as, for 

 example, in the case of the Aulosphserida and some Sphserellaria). 



117. The Lattice-Spheres of the Spumellaria. The lattice-spheres or Sphseroid 

 skeletons of the SPUMELLARIA exhibit in spite of their simple type of structure, an extra- 

 ordinary variety in the formation of the lattice-work and radial apophyses, so that in the 

 systematic portion of this work no less than one hundred and seven genera and six hundred 

 and fifty species are distinguished ; these are united in one suborder, the Sphseroidea 

 (pp. 50-284, Pis. 5-8, 11-30). It maybe divided into two main divisions, the 

 Monosphcerida with a single primary lattice-sphere (Pis. 12-14, 21, 26, 27), and 

 Pliosphcerida (or Sphseroidea concentrica) whose skeleton consists of two or more con- 

 centric lattice-spheres united by radial bars. The latter are subdivided into Dyosphserida 

 with two concentric lattice-spheres (Pis. 16, 19, 20, 22, 28) ; Triosphserida, with three 

 spheres (Pis. 17, 24, 29); Tetrasphserida, with four (Pis. 23, 30) ; Polysphasrida, with 

 five or more (Pis. 15, 23); and Spongosphserida, with spongy lattice-spheres (Pis. 18, 

 25). A special group is made up of the simple lattice-spheres of the social Collosphserida 

 (or Sphseroidea polyzoa) (Pis. 5-8) ; these are usually more or less irregular, and charac- 

 terised by the development of peculiar tubular processes ; the latter are generally wanting 

 in the Sphseroidea monozoa, whose lattice-shell is very regularly formed. This distinction 

 is interesting and important, inasmuch as the regular lattice-spheres are explained by the 

 independent development of the free-swimming Monozoa, whilst the irregular spheres 

 are due to the mutual dependence of the social Polyzoa. 



118. The Lattice- Spheres of the Acantharia. The lattice-shells or Sphseroid skeletons 

 of the ACANTHARIA are immediately distinguishable from those of all other Radiolaria by 

 their centrogenous development and the central union of the radial spines by which they 

 are supported; the only exception is furnished by the remarkable genus Cenocapsa 



