272 D1ADEMAT1DAE. 



DIADEMATIDAE. 



Family Diadematidae Peteks, 1853. Monatsb. Akad. Berlin, (emend.) 



ASTHENOSOMA. 



Asthenosoma Grube, 18C7. Jahresb. d. Sillies. Ges. f. Vat. Cul. 



Test flexible ; ambulacral and interambulacral areas composed of narrow 

 plates, in which calcareous deposits are limited to the median ambulacral 

 and interambulacral spaces, the remainder of the plate being more or less 

 membranous. The calcareous part of the plates is pistol-shaped, the but end 

 turned towards the actinostome in the interambulacral area, and turned in the 

 opposite direction in the ambulacra; they lap slightly along the median line 

 in both areas, and turning in opposite directions, give the test a great degree 

 of mobility. This character of lapping of the plates in opposite directions 

 in the two areas was thus far only known among the Palaechinidae (Melo- 

 nites, etc.). The tentacles are pointed near abaetinal area, as in the Dia- 

 dematidae. The abaetinal system resembles that of other Diadematidae, has 

 a small anal system, and approaches more the structure of that of Centroste- 

 phanus. There is in each area a principal vertical row of large perforate tuber- 

 cles; the rest of the plate is occupied by smaller tubercles, similar in struc- 

 ture, arranged in an irregular horizontal line, — a mode of arrangement thus 

 far not found among the Diadematidae. The poriferous zone is not formed 

 of independent plates, the large ambulacral plates are themselves perforated 

 for one pair of pores, the two other pairs passing through small scales, 

 resembling the ten buccal plates of Echini of the membranous part of the 

 large plate ; they form three vertical rows of pores, reminding us some- 

 what of the arrangement of the pores of Hipponoe ; the two inner vertical 

 rows passing through the membranous part of the large plate are placed 

 close together ; the other vertical row is well separated, placed nearer 

 the interambulacral zone, their structure recalling remarkably the struc- 

 ture of the poriferous zone of Palaechinidae. 



