vni ECHINODERMATA MORPHOLOGY OF SKELETON 347 



are of unequal length owing to the shifting of the apical system, be viewed from 

 the oral side, the ambulacra still form a regular, or almost regular, five -rayed 

 star round the central oral aperture or peristome. Where, however, as in the 

 Spatangoida, the peristome with the mouth has moved from the centre of the oral 

 surface (on which the Echinoids creep), and is shifted more or less anteriorly, and 

 finally, where in the Pourtalesia it comes to lie quite on the anterior ambitus, the parts 

 taken by the five ambulacra in the formation of the oral surface are necessarily very 

 different. The unpaired anterior ambulacrum (III) and the two anterior and 

 lateral ambulacra (II and IV) shorten and form an ever smaller portion of the 

 whole ambulacra! area of the oral (ventral) surface, in proportion as the peristome 

 with the mouth shifts forward. They form together the trivium. Conversely, the 

 two posterior radii at the same time lengthen and form an increasingly large 

 portion of the ambulacral area of the ventral surface. They form the bivium. The 

 length of the ambulacra of the trivium and the bivium in the apical direction is of 

 course determined by the position of the apical system. If this system shifts for- 

 ward, the trivium is shortened apically ; if backward, the ambulacra of the trivium 

 (especially the anterior unpaired ambulacrum) are lengthened, while those of the 

 bivium are shortened. This grouping of the ambulacra into an anterior trivium 

 and a posterior bivium is especially clear on the apical surface of those Spatangoida 

 which have a diffused apical system, e.g. the Collyritidce and Pourtalesiidce (cf. pp. 

 324, 325). Since the apical ends of the ambulacra are always in contact with the 

 radial plates of the apical system, and since, further, in the diffused apical system 

 the two posterior radials I and Y, which are separated from the anterior, are 

 shifted posteriorly, the apical ends of the two posterior ambulacra (the bivium) are 

 also necessarily separated from the three anterior ambulacra (the trivium) by a con- 

 siderable space (Fig. 284, p. 325). 



In the Palccechinoidea, and among the Euechinoidea in the Cidaroida, the Diade- 

 matoida, nearly all Holeetypoida, and many Spatangoida, the ambulacra throughout 

 their whole courses have a similar structure, and are similar!} 7 provided with pores. 

 In the Clypeastridce and many Spatangoida, however, the ambulacra are modified on 

 the apical side in a characteristic manner ; they are petaloid, each ambulacrum forming 

 a petalodium (Figs. 233, 234, p. 292 ; 236, p. 294, and 306). Such a petalodium 

 arises by the divergence of the two rows of large double pores of each ambulacrum 

 from one another immediately on leaving the apex, and their reapproximation and 

 junction before they reach the ambitus. The two rows of pores of each petalodium 

 make a figure like a lancet-shaped leaf, and the five petaloids together form round 

 the apex a graceful rosette of leaves, which recalls the petals of a flower. On the 

 remaining plates of the ambulacra, i.e. those not forming the petalodium, the pores 

 are single and small ; they are, further, few in number and scattered. Between 

 the regular ambulacra and those which have apical petaloids there are many transi- 

 tion forms, occurring often within one and the same family. One of these transi- 

 tions is specially frequent ; the two rows of pores of a petalodium do not unite 

 at their oral ends but remain open. The ambulacra are then called sub-petaloid. 

 Such petaloids are often very long. 



The petaloids often sink in (Fig. 236, p. 294), and then, not infrequently, serve 

 as brood cavities, or marsupia, for containing the young. 



Just as the ambulacra occasionally form petaloid rosettes round the apical system, 

 so, in the family of the Cassidulidw (sub-order Cassiduloida of the order Spate n- 

 yoida), can they form rosettes of so-called phyllodes round the peristome (Fig. 307). 

 The five phyllodes, in which the well-developed double pores lie thickly crowded 

 together, sink in, while the five interradial marginal plates of the peristome between 

 them are contrariwise bulged out. The five interradial cushions form, together with 

 the five radial phyllodes, what is called a floscelle. 



