ORIENTATION. 33 



developed in all the irregular Echini. The ambulacra fall into a posterior pair, the bivium of 

 Loven, and an anterior set of three, consisting of the right and left anterior ambulacra and 

 the odd anterior ambulacrum, the trivium of Loven. 



It has been claimed that in regular Echini the madreporite is the only structure on which 

 orientation could be based (A. Agassiz, 1881, p. 7). Wliile this is apparently true in many 

 Echini, I have attempted to show under the discussion of ocular plates that in many regular 

 modern Echini a bilateral symmetry is expressed and orientatioii obtained by the order in which 

 ocular plates reach the periproct. To state the case in brief: in the young of apparently all 

 modern regular Echini, the ocular plates are exsert." The same condition exists in the adult 

 of many species. On the other hand, many species have in the adult one or more plates insert, 

 or reaching the periproct, either as a typical character or as a variation. On the basis of ob- 

 servations on 50,000 specimens, the evidence is that the first oculars to become insert are the 

 plates of the bivium, next the plates of the posterior pair of the trivium, and last, if at all, the 

 anterior odd plate of the trivium. The order of reaching the periproct is I, V, or V, I, then IV, 

 II, III. This order is very closely adhered to, as shown later in tabulated form. The ocular 

 plates therefore in many regular Echini express a bilateral symmetry in this group, and an 

 orientation passing through ambulacrum III and interambulacrum 5, the plane of symmetry 

 adopted by Loven. 



As studied from the ventral view, Loven (1874) showed graphically that the size and 

 character of the primordial ambulacral plates give .excellent data by which to orient a sea- 

 urchin, at least in the Irregulares and often in the Regulares as well. He showed that of the 

 ten basicoronal ambulacral plates in (typical) clypeastroids and spatangoids, the la, Ila, 1116, 

 IVa, \b are larger, and in spatangoids possess two separate pairs of pores, or two separate 

 single pores, indicating two tube-feet. On the contrary, the 16, 116, Ilia, IV6, Va are smaller, 

 and in the spatangoids bear only a single tube-foot, Collyrites (Plate 3, fig. 15). This system 

 of alternation has but this one combination, a sort of key, by which the axes can be ascer- 

 tained with entire certainty, as shown in numerous genera and species (Loven, 1874). Mr. A. 

 Agassiz shows the same characters in his splendid work on Panamic Echini in numerous 

 spatangoids as well as in many regular Echini. 



Loven (1892) showed that in the primordial ambulacral plates on the peristome in young 

 Goniocidaris (Plate 2, fig. 1) the same system of alternation of large and small plates prevails. 

 He (1874) also showed that it exists in the primordial plates surrounding the mouth in adult 



1 Mr. Agassiz (1881, p. 7) objected to Loven's orientation as it does not take account of the madreporite in relation 

 to the spiral system of development of plates. The spiral system was first suggested by Professor L. Agassiz (1834) and is 

 considered by A. Agassiz in several publications (1864, p. 12; 1874, pp. 640, 722, 724; 1881, p. 7; 1892, p. 95). I regret 

 that I have not succeeded in recognizing the spiral arrangement of plates in Echini. 



2 It is possible that in Aspidodiadema and close allies the oculars are insert from their earliest stages. 



