DESMOSTICHA. 



Suborder Desmosticha IIaeckei,, Entwiekel. Gesch., 1866. (emend.) 



The order of Desmosticha contains the so-called Regular Echini of Albin 

 Gras, the Endocycla of Wright. The Echini belonging to this order are all 

 more or less circular in outline. The anal system is surrounded and com- 

 pletely enclosed by the genital and ocular plates. The actinostome is central. 

 The poriferous zone is composed of pairs of pores, extending from the abac- 

 tinal system to the actinostome, either as simple vertical rows or forming 

 numerous disconnected arcs. The jaws are highly developed and quite 

 complicated, supported upon prolongations (auricles) of the ambulacra! or 

 interambulacral plates of the edge of the actinostome, which may be con- 

 nected to form either arches or disconnected supports. A longitudinal axis 

 is indicated by the position of the madreporic body. The Desmosticha, com- 

 pared with the Perischoechinidae, (lifter in having only ten double rows of 

 ambulacral and interambulacral plates, five called ambulacral and five inter- 

 ambulacral. As far as known, they are all provided with branching gills 

 extending through openings of the coronal edge of the buccal membrane, 

 corresponding to cuts more or less marked in the outline of the test of the 

 actinal system. The digestive system is a simple alimentary canal, arising 

 above the jaws, suspended in a cavity, like a thin band, by mesenteries 

 attached to the interambulacral system, arching over the ambulacral system. 

 The ovaries in the breeding-season are large, they nearly fill the whole 

 space not occupied by the digestive system. Stone canal well developed. 

 Ambulacral tentacles pointed or provided with suckers. With the excep- 

 tion of the auricles for the support of the jaws and the madreporic funnel 

 of the stone canal, there are no pillars or partitions of any kind in the 

 interior of the test. Actinal system covered by a flexible buccal membrane 

 attached to jaws. Actinostome opening within the teeth, and leading 

 directly to the small part of the alimentary canal. 



The tubercles carrying the spines form vertical or horizontal rows on the 

 ambulacral and interambulacral plates. The spines are comparatively large 

 compared to the size of the test, and less numerous than in the other sub- 

 orders of recent Echini. 



