66 HAWAIIAN AND OTHER PACIFIC ECHINL 



some Arbacias), or narrow, curved perforated plates (Dialithocidaris), or more 

 irregular, wider, rough perforated plates {Arbacia stellifera). It is doubtful 

 how much reliance can be placed on these calcareous particles for specific 

 characters. 



The gills of the Arbaciadae contain more or less numerous perforated 

 plates (Pis. 47, figs. 11, 19; 49, fig. 28), which apparently increase in 

 number and complexity with age. In their simplest condition they are 

 smooth, flat plates with few, large perforations (PI. 47, fig. 11), but they 

 often have numerous knobs or projections on the surface and are perforated 

 with many small holes. Sometimes they become still more complex, and 

 are irregular masses of calcareous tissues (PI. 47, fig. 19). When very 

 large and well developed, they frequently carry pedicellariaB. 



The general arrangement of the internal organs of the Arbaciadae is 

 shown in the figures given of Coelopleurus (PI. 44, figs. 1,2). The reproduc- 

 tive organs form finely divided tufts in each interambulacrum, the size of 

 which varies of course with the sexual condition of the specimen. The 

 oesophagus is moderately long and connects abruptly with the remarkably 

 large, flat, stomach-intestine, which is very slightly undulated. The upper 

 intestine is more undulated and shows five distinct interradial " pockets." 

 It ends in a short but wide rectum. 



The Genera and Species of Arbaciad^. 



This easily recognized family contains seven apparently valid genera, all 

 of which contain recent species. These genera are distinguished from each 

 other by the primary spines, the tliickness of the test, the number of anal 

 plates, the structure of the ambulacral plates, the absence or presence and 

 arrangement of non-articulated spines and of secondaries, and the pedicel- 

 larioe. A convenient grouping of the genera according to these characters 

 may be made as follows : 



]*riinary spines short, never mucli longer than diameter of test; spha?- 

 ridial pits wanting or one present at peristome in each ambulacrum. 

 Test thick and solid as in most regular Echini ; primary spines 

 cylindrical or flattened; valves of tridentate pedicellaria; not re- 

 markably flattened; valves of ophicephalous pedicellariae not ex- 

 traordinarily constricted nor witli unusually expanded apophysis. 

 Abaclinal surface with numerous articulated primary sjiines, 

 which are more or less cylindrical, though they may be flat- 

 tened near tip. 



