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



characteristic of this suborder that they all usually lie in the horizontal median plane of 

 the lenticular shell, arising from its equatorial margin. The Larcoidea (Pis. 9, 10, 

 49, 50) show a great variety in the number and arrangement of their radial main- 

 spines, which in the different families of this suborder stand in direct causal relation to 

 the various forms of growth of the shell ; usually the primary main-spines lie either in 

 the three different dimensive axes, at right angles to each other, whose differentiation is 

 characteristic of the lentelliptical Larcoid shell (34, 122) or in definite diagonal axes, 

 which cut the former obliquely. The radial spines of the SPUMELLARIA are never united 

 in the centre of the body, but arise separately from the surface of the primary central 

 lattice-shell (medullary shell), more rarely from one of the secondary (cortical) shells, 

 which enclose it. Their form is originally three-edged (sometimes pyramidal, sometimes 

 prismatic); the cause of this is to be found in their origin from the nodal points of the 

 lattice-shell, whose meshes are primitively hexagonal ; hence three trabeculse unite in 

 each nodal point, and are produced into the three edges of the spine. Very commonly, 

 however, the spines are round (conical or cylindrical), more rarely polygonal. The three 

 edges are often delicately toothed, not unfrequently spirally twisted around the axis of 

 the spine (PI. 21, figs. 1, 12). 



137. Radial Spines of the Acaniharia. The radial spines of this legion have a much 

 greater significance than in the other three classes of Racliolaria, since here alone they 

 are the primary determining factors in the skeletal structure, and grow outwards from 

 the middle of the central capsule. This centrogenous origin of the radial spines is as 

 characteristic of the ACANTHARIA as their chemical constitution, which is not siliceous 

 but acanthinic ( 102). Furthermore, their form is in most cases so peculiar that even an 

 isolated ACANTHARIAN spine can be generally distinguished from one belonging to either 

 of the other three legions. In the great majority of the ACANTHARIA (all Acanthonida 

 and Acanthophracta) twenty radial spines are constantly present, which, disposed 

 according to a definite geometrical law, make up the skeleton (compare 110 above and 

 p. 717). The twenty spines are generally simply apposed to each other in the centre 

 (either by the surfaces or the edges of their pyramidal base); more rarely they are 

 completely united and form a single star-like piece of acanthin (Astrolithium). Very 

 rarely (Acanthochiasma) each two opposite spines are united so that ten diametric 

 bars cross in the middle of the central capsule. Whilst in the great majority of 

 ACANTHARIA these twenty radial spines are present, the small group Actinelida is 

 characterised by the possession of an inconstant, often very large number, sometimes over 

 one hundred. Among these Actinelida are probably to be found the stem-forms of 

 the whole legion. The variously modified spines of the ACANTHARIA may be grouped in 

 three main categories: (l) round (cylindrical or conical) ; (2) four-edged (prismatic or 

 pyramidal); (3) two-edged (leaf- or sword-shaped). The latter very commonly bear two 



