62 ROBERT TRACY JACKSON ON ECHINI. 



Eucidaris tribuloides and thouarsii. Stewart's organs are described in Asthenosoma by P. 

 and F. Sarasin (1888), and in several echinothuriids by A. Agassiz and Clark (1909). External 

 gills are developed as five pairs of branched appendages which are outgrowths of the oral integu- 

 ment (text-fig. 55, p. 83). They are characteristic of the Centrechinoida and by inference the fos- 

 sil Holectypina. Their presence is marked by the indenting of the interambulacral basicoronal 

 plates so that they are recognizable in fossils. These peristomal gills, as I would call them 

 for distinctiveness, are interambulacral in position in contradistinction to Stewart's organs, 

 which are ambulacral or radial in position. In clypeastroids and spatangoids, as well as par- 

 tially in some of the Centrechinoida (Arbacia), the function of respiration is maintained by the 

 dorsal ambulacral tentacles, which have lost their function as locomotive organs. These I 

 would call in distinction ambulacral gills. We do not and probably cannot know definitely 

 what respiratory organs existed in Palaeozoic Echini. Branchial slits for peristomal gills I 

 have found no evidence of, though carefully looked for. There is no evidence of specialization 

 of tube-feet as ambulacral gills, though they may have performed that function in part. As 

 the primitive Cidaridae have the Stewart's organs so well developed, it is not unlikely that 

 these organs were the respiratory organs of all Echini in Palaeozoic times. 



The Interambulacrtjm. 



The interambulacrum in Echini functions chiefly as a space filler and a bearer of spines 

 and pedicellariae. The spines serve for protection and more or less in locomotion, and pedi- 

 cellariae as grasping, cleansing, and protective organs. In spite of this secondary physiological 

 importance, the interambulacrum forms a large part of the test of the sea-urchin in most types, 

 and is of very great interest, especially in Palaeozoic genera. 



The interambulacral plates originate in direct contact with the ocular plates and quite 

 independently of the genitals. The young last added plates may always be found in direct 

 contact with the oculars in Palaeozoic and later Echini, and their independence of the genitals 

 is proved in cases where no genital reaches the interambulacrum, as in Bolhriocidaris archaica 

 (Plate 1, fig. 2), the posterior area in spatangoids (text-figs. 174, 175, p. 149), the aberrant 

 Arbacias (Plate 4, figs. 11, 12), and Tripneustes (Plate 6, fig. 4). As any given interambulacrum 

 comes in contact with two oculars, one on either side, and as at the base of these two oculars new 

 interambulacral plates originate (Melonechinus, Plate 56, fig. 6, and numerous other figures), it 

 seems that an interambulacrum may theoretically be considered as composed of two halves, 

 one half-interambulacrum being associated with the ocular and ambulacrum on one side, and 

 the other half being associated with the ocular and ambulacrum on the other side. In other 

 words, the corona may be conceived as made up of five areas, each surmounted by an ocular 

 plate, an ambulacrum and adjacent half-interambulacrum on either side making up each 



