* THE PENTAMEROUS SYSTEM AND VARIATION. 47 



the same diameter of 110 mm. The interambulacra are normal in appearance superficially, 

 but at the mid-zone they all measure 27 mm. in width, that is, they are narrower than the 

 ambulacra. In a normal specimen of the same size the five interambulacra measure each 38 

 mm. in width when the ambulacra measure 29 mm. In the six-rayed specimen, therefore, evi- 

 dently the space gained to add the extra ambulacrum and interambulacrum is attained by 

 building ambulacra of practically the usual width, but narrowing all the interambulacra 

 equally to much less than the usual width. This emphasizes the conclusion gathered from 

 normal Echini that the interambulacrum is essentially a space filler and adapts itself to fill 

 what space is available between the ambulacra which are the more essential structures. There 

 are twelve cuts in the test ventrally for the peristomal gills, and the lantern has six teeth and 

 all its other parts in sixes or multiples of the same. In the narrowed interambulacral areas 

 the tubercles (Plate 6, fig. 3) occupy the same distance apart as in ordinary pentamerous speci- 

 mens, but owing to the narrowness of the areas there are fewer tubercles on each plate. This 

 case with the Arbacia lixula previously considered (p. 40) demonstrates that the distance 

 apart and size of tubercles is the species feature and not the actual number of tubercles on a 

 given plate. 



The same hexamerous structure as just described occurs in a Strongyloccntrotus drobachien- 

 sis from Dumpling Islands, North Haven, Maine (R. T. J. Coll., 838, Plate 8, fig. 3). This 

 choice specimen measures 53 mm. in diameter, and 25 mm. in height. There are six ambulacra 

 and interambulacra complete, six oculars and six genitals, but genitals 3, 6 are fused, with 

 ocular VI between them exactly as in Plate 6, fig. 2. The ambulacra at the mid-zone measure 

 11 mm. in width and the interambulacra 12 mm. In a normal pentamerous specimen of the 

 same size the ambulacra measure 12 mm. and interambulacra 19 mm. in width. It is clear, 

 therefore, that as in the Tripneustes, the space for the sixth areas is attained mainly bj' 

 narrowing the interambulacra and retaining ambulacra of practically the usual width in all 

 areas. Ventrally there are twelve cuts for the peristomal gills. Unfortunately the jaws are 

 absent, but they were doubtless hexamerous as all other parts of the sea-urchin have this 

 arrangement. I was startled by finding this same structure for a third time in a small 

 Strongijlocentrotus drobachiensis, also collected at Dumpling Islands (Plate 7, figs. 7, 8). The 

 specimen is 14 mm. in diameter and 6 mm. in height; it is perfectly shaped without any dis- 

 tortion. There are six ambulacra and interambulacra throughout the corona, six oculars and 

 genitals, but as in the two other cases, genitals 3, 6 are fused, with ocular VI between them, 

 as seen in Plate 7, fig. 7. It is certainly most extraordinary that this parallel structure 

 should exist in three specimens, and indicates what I have elsewhere pointed out, how very 

 definite even extremely rare variation may be. Ventrally there are six teeth, twelve pri- 

 mordial ambulacral plates around the mouth and twelve cuts for the peristomal gills (Plate 7, 

 fig. 8). The three similar specimens of hexamerous Tripneustes and Strongylocentrotus just 



