58 ROBERT TRACY JACKSON ON ECHINI. 



opposite direction. From this it occurs that the pores of each pair occupy usually an inclined 

 position, and the outermost pore of a pair is a little higher than the inner pore, Hyattechinus 

 (Plate 23, fig. 1). It may be on a line with the inner pore, as in Melonechinus (Plate 61, fig. 8), 

 but it is rarely lower than the inner pore in regular Echini. At the ventral border of the test 

 the pores of each pair lie much more nearly vertical than at the mid-zone in Strongylocentrotus 

 and Arbacia. In the latter the pores of the two pore-pairs adjacent to the median spheridium 

 are actually superposed, as in Bothriocidaris. It is to be noted also that the pores on the 

 peristomal ambulacral plates, as in Eucidaris (Plate 2, fig. 6), Phyllacanthus (Plate 2, fig. 18), 

 Phormosoma (text-fig. 43, p. 80), and Strongylocentrotus (text-fig. 50), are nearly or quite 

 vertically superposed as in the young and in the primitive Bothriocidaris. Also it may be 

 shown that in very young, newly formed plates next the ocular, of Goniocidaris (Plate 2, fig. 

 4), the plates are high, not low, and the pores are nearly or quite superposed. This is in a 

 measure like the young of Goniocidaris (Plate 2, fig. 2) or adult of Bothriocidaris. I have seen 

 the same character in very young plates of Eucidaris tribuloides close to the ocular. 



In many clypeastroids, as Echinarachnius parma (Plate 8, fig. 4), the pores revolve through 

 an angle of more than 90 degrees from the vertical, so that the outer pore of each pair lies lower 

 than the inner pore. In many spatangoids the pores ventrally, and in most of the other ambula- 

 cral plates, are superposed as in the embryo and Bothriocidaris, but in the petaloid areas they 

 are nearly or quite horizontal. In Metalia pectoralis (text-fig. 8) the plates in ambulacrum III 

 are high, without petaloid expansion, and the pores are superposed in the middle of the plate; 

 in the four other areas the plates are relatively lower, petaloid dorsally, and in these areas at 

 least the pores are horizontal, not superposed, and not in the middle of the plates. From this 

 it seems that the relative position of the pores represents a greater or less departure from the 

 primitive, and that a more primitive or more specialized relative position of pores occurs at 

 definite parts of the test in the same specimen. 



In adult Bothriocidaris, Palaeechinus (text-fig. 15), and in the young, as seen in Gonio- 

 cidaris, Strongylocentrotus (Lov6n, 1892) ; also in the young, as shown at the ventral border 

 of the test, in Maccoya (text-fig. 16; Plate 33, fig. 1) and Lovenechinus (Plate 42, fig. 1), we 

 find that the pore-pairs in succeeding plates lie over one another in a continuous or uniserial 

 system. This arrangement is certainly primitive. It is to be observed that in this condition 

 all tube-feet extend to the ground from one continuous line, and when the plates are low, that 

 they are proximally closely crowded as in Cidaris. FVom this condition of uniserial pores and 

 tube-feet, we pass in many independent series to types in which the pore-pairs are biserial, 

 triserial, or polyserial in each half-ambulacrum. This is accomplished by alternate plates 

 failing to reach the interambulacrum in whole or in part, thus producing a biserial arrangement, 

 Maccoya (text-fig. 16; Plate 33, fig. 1) and Lovenechinus (text-fig. 18; Plate 42, fig. 2). In 

 addition isolated plates may be introduced to make a triserial arrangement of pores, Melon- 



