348 COMPARATIVE ANATOMY CHAP. 



The anterior unpaired ambulacrum in many exocyclic Echmoidea differs greatly, 

 th in s^Tlnn the number, arrangement, and form of its pores, from the other 



both in s 



a 

 V 



FIG. 307.-0ral perisome of Cassidulus pacificus, Ag., with the five phyllodes (after Loven). 



four. This variation in the anterior ambulacrum is found almost exclusively in the 

 order Spatangoida, especially in the Cassiduloid family Plesiospatangidce and in the 

 sub-order Spatangoidea (here especially, and, to a very marked degree, in the family 

 of the Spatangidce). 



((/) Special Modifications of the Interradii. 



We can here only point out certain conditions occurring in the order Spatan- 

 goida. 



In the sub-order Spatangoidea an extraordinary asymmetry of the two 

 posterior interradii 1 and 4 prevails (cf. Fig. 302, p. 342). The right posterior in- 

 terradius 1 is always so modified near the peristome that two plates fuse, thus con- 

 trasting with the left posterior interradius, which remains only slightly if at all 

 modified. This fusion takes place either between the second and third plates of the 

 row la, or the two second plates of rows la and 1&, or the second and third plates 

 of row b and the second plate of row a. In the last case, the second plates of the 

 two rows of interradius 4 are also fused. 



Since, in the Spatangoida, the peristome, with the mouth, is shifted forward on 

 the oral surface, the posterior unpaired interradius occupies a considerable portion of 

 the ventral surface (and this is also the case in the Cassiduloidca with mouth shifted 

 forward). It is often somewhat bulged out, and the region occupied by it on the 

 oral side is known as the plastron. It takes part in the limitation of the peristome 

 by means of a single crescent-shaped plate, which is known as the labrum in those 

 forms which have a projecting under-lip to the transverse peristome (cf. Fig. 302, 

 p. 342). In many Spatangoida the labrum is followed posteriorly by two large sym- 

 metrically arranged plates (sternum), which again are followed by two smaller but 



