718 THE VOYAGE OF H.M.S. CHALLENGEfi. 



morphology of all Icosacantha, it is the most profitable way to retain constantly in 

 mind for comparison the figure of a terrestrial globe with its axis and zones. The 

 axis of the globe is the spineless axis of all Icosacantha, around which all twenty spines 

 are symmetrically disposed ; it is perpendicular to the bisecting equatorial plane, 

 in which lies the middle of the five parallel zones ; therefore the four spines, crossed 

 perpendicularly in this equatorial plane, are called the equatorial spines (cl to c4 in the 

 figures of Pis. 131-140); often, and mainly in the family Quadrilonchida (PI. 131), 

 these four equatorial spines are much larger or of a peculiar form, different from that 

 of the sixteen other sj)ines. Each pair of the four equatorial spines lies in 

 one equatorial axis, and this latter is perjoendicular to the crossing axis, in which lies 

 the other pair of opposite spines. We may regard these two equatorial diameters, 

 perpendicular one to another and to the spineless axis, as the two perradial axes 

 or primary axes. Correspondingly the two meridian planes, which are determined 

 by one perradial axis and the spineless axis, may be called the two primary or perradial 

 meridian planes. 



The globe is divided by the equatorial plane into two equal halves, the northern 

 and the southern hemisphere. In each hemisphere there are disposed quite sym- 

 metrically eight radial spines, the distal ends of which fall in two parallel circles, a 

 larger tropical circle (nearer to the equator) and a smaller polar circle (nearer to the 

 pole of the spineless axis). Therefore we call the four spines of the former the 

 "tropical spines" and the four spines of the latter the "polar spines." The angle 

 between the former and the equatorial plane is about 30°, the angle l:)etween the latter 

 and that plane about 60°. 



The eight polar .spines (four northern and four southern) lie in the same two 

 meridian planes as the four equatorial spines. Therefore in each of these two perradial 

 planes lie six radial spines, opposite in pairs ; two equatorial and four polar spines. 

 Commonly all eight polar spines are of the same size and form ; and often they are also 

 equal to the eight tropical spines ; but in some cases {e.g., in some species of Quadrilon- 

 chida) they are much smaller than the twelve other spines, and sometimes even rudi- 

 mentaiy. In all figures of the Pis. 131-140 (and also in my Monograph, 1862, 

 Taf. xv.-xxii.) the polar spines of the northern circle are marked by the characters 

 al to ai, the polar spines of the southern circle by the characters el to ei. In 

 the first perradial meridian plane lie al and «3, el and e3, in the second a2 and a4, 

 e2 and e4. 



The eight tropical spines lie between the eight polar and the four equatorial 

 spines, four in each hemisphere ; their distal points fall in two parallel circles, which 

 correspond exactly to the two tropics of the globe. Therefore the four northern 

 tropical spines may be called " canceral spines " (as their ends faU in the Tropic of 

 Cancer) and the four southern correspondingly " capricornal spines " (as theii" points lie 



