:U8 COMPARATIVE ANATOMY chap. 



The anterior unpaired ambulacrum in many exocyclic Ediinoidea differs greatly, 

 liotli ill sliaiii' and in tlie iiuiuher, arrangement, and form of its pores, from the other 



V 



Fig. 307.— Oral perisome of Cassidulus pacificus, Ag., with the live phyllodes (after Lovfen). 



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

 order Spataiujoida, especially in the Cassiduloid femily Plesiospaf.angida' and in the 

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

 of the Spatangidce). 



(g) Special Modifications of the Interradii. 



We can here only point out certain conditions occurring in tlie order Sjxtian- 

 goida. 



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

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

 terradius 1 is always so modified near the peristome that two plates fuse, tints 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 jdates of the 

 row \a, or the two second i)lates of rows la and \b, or the second and third plates 

 of row h and the second plate of row n. In the last case, the second plates of the 

 two rows of interradius 4 are also fused. 



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

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

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

 forward). It is often somewhat bulged otit, 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 j)late, 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 Spatangokla the labrum is followed posteriorly by two large sym- 

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



