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



The third mode of junction, by central concrescence of all twenty spines, was 

 formerly regarded by me as an important peculiarity, sufficient for the separation of 

 subfamilies and genera (Monogr. d. Kadiol., 1862, pp. 399, 401 ; Prodromus, 1881, 

 p. 466). But I found afterwards that in many species where the twenty spines 

 commonly remain separated, accidentally they grow perfectly together and form one 

 single piece of acanthin a starrulet with twenty rays. Therefore I now think it is 

 more natural to divide those species only into different subgenera. 



A fourth and a very different mode of junction, quite sufficient for the distinction of 

 different families, is the concrescence in pairs of every two opposite spines, lying 

 in one diameter (in Acanthochiasma and Chiastolus). Here we obtain a number of 

 " diametral spines " (each composed of two originally opposed radial spines) and all 

 these diametral spines are crossed loosely near the central point of the body without 

 any solid and permanent apposition (Chiastolida). However, in some species of this 

 peculiar family the central part of the diametral spines is twisted like a screw or spirally 

 convoluted (PI. 129, figs. 2, 3). 



Tlie Form of the Radial Spines in the ACANTHARIA is extremely varied, and con- 

 stitutes the main characters for the distinction of nearly four hundred species. But 

 all these different forms may be reduced phylogenetically to three different fundamental 

 forms : (a) the cylindrical (with circular transverse section), (V) the two-edged (with 

 elliptical or lanceolate transverse section), and (c) the four-edged (with square transverse 

 section). No doubt the first (a) is the original primitive form, from which the two 

 others are secondarily derived. Triangular spines never occur in the ACANTHARIA, 

 whilst, however, they are common in the S p h se r e 1 1 a r i a. The first and original 

 form, the cylindrical spine, is either a true cylinder of equal thickness in its whole 

 length, or it is more or less conical. Rarely the spine is in the distal half spindle- 

 shaped, and thicker than in the basal half. The second form, the two-edged spine, is 

 more or less compressed from two opposite sides ; its two edges are either more blunt, 

 rounded, or more acute, sharp ; its transverse section in the former case is elliptical, 

 in the latter case lanceolate or rhomboidal. Sometimes the two edges are broader 

 and in the form of two thin opposite wings. The two-edged spines may be occasion- 

 ally shorter, triangular or lanceolate, at other times longer sword-shaped or linear. 

 The third form, the four-edged spine, has constantly a square transverse section ; 

 the sides of this square are either even or concave ; in the latter case the four edges 

 are broadened and wing-like, but in the former case not. The quadrangular spines are 

 either prismatic (of equal breadth throughout their whole length) or pyramidal 

 (becoming gradually thinner towards the distal apex). 



The Apex of the Radial Spines, or their free distal end, is in the majority of ACAN- 

 THARIA simple, conical. In the minority it is either truncated or bifid, or four-sided 

 pyramidal, often with two, rarely with four prominent parallel teeth. In some forms 



