142 HAWAIIAN AND OTHER PACIFIC ECHINI. 



That of Sperosoma (PI. 62, fig. 4) shows some noticeable resemblances to 

 the arrangement in Echinothrix (PI. 60, fig. 3), though it has certain 

 peculiarities of its own, in addition to the long, slender oesophagus. 

 There can be no question that the general arrangement of the canal in 

 Asthenosoma (PI. 62, fig. 1) is quite as near what we find in Astropyga 

 (PI. 61, fig. 3) as it is to that of any genus of Echinothurids. 



A comparison of the lanterns and perignathic girdles figured on Plate 63 

 with those of the three genera of Diadematidoe (PI. 60, fig. 4 ; PI. 61, figs. 2, 

 4) reveals two very important differences. In the Echinothurids, the apo- 

 physes, whether conspicuous as in Phormosoma (PI. 63, fig. 3), or relatively 

 inconspicuous as in Asthenosoma (PI. 63, fig. 2), are single, in keeping with 

 the presence of the single primordial interambulacral plate, while in the 

 Diudematids they are paired. In the Echinothurids (PL 63), the Stewart's 

 organs, though very different in appearance from those of the Cidaridaa, are 

 well developed, 1 while in the Diadematids they are either wholly wanting 

 or are reduced to mere rudiments as shown in Echinothrix (PI. 60, fig. 4). 

 Aside from these two points, the differences between the girdle and lan- 

 tern of Phormosoma and those of Asthenosoma are as great as those 

 which distinguish Phormosoma from Astropyga, or Echinothrix from Micro- 

 pyga. We find there is noticeable individual diversity in certain points, 

 such as the bulk of the auricles and the amount of forking at the end of 

 the compasses, so that these features are of little value in determining 

 generic limits. 



The Spines, Pedicellari^:, Sph2eridia, and Spicules. 



Plates 64-67. 



The spines of the Echinothuridte show so much diversity of form, aside 

 from the differences between primary, secondary, and miliary spines, that 

 they deserve a special paragraph. They may be conveniently classified 

 under four heads, — naked spines, sheathed spines, glandular spines, and 

 hoofed spines. The naked sjmies are the ordinary spines, more or less 

 abundant on all parts of the test; they may be slender or stout, pri- 

 maries, secondaries, or miliaries ; they may be either sharp or blunt, or 

 rarely, widened and flattened at tip, long or short, smooth or rough ; they 



1 Those writers who have said that Stewart's organs are rudimentary or wanting in certain 

 Echinothurids have either had poor material or have overlooked these long, slender outgrowths. 

 Or else there is more individual diversity than would naturally be expected. 



