ANAL SYSTEM. 



In all the Desmosticha the anal system is enclosed by the genital and 

 ocular ring. It consists of a series of polygonal plates, usually irregularly 

 arranged, becoming smaller towards the anal opening, round which they are 

 minute, lapping so as to close the opening completely (Pi. VII f. 19; PI. 

 VIII. f. 24). In the Diadematidae the anal sj'stem is quite large, covered 

 by a nearly bare membrane (PL III a .f. .;). strengthened round the base by 

 plates (PL IP.f. 6, 7). The anal opening is placed at the extremity of a 

 conical tube projecting tar beyond the general level of the anal system 

 (PL IP.f. 6; PL IIP./. 1. 4 ; PL IIP./. 5). As I have shown in my Em- 

 bryology of Echinoderms,* the anal system in the very j^oungest stages is 

 covered with only a single plate (PL VIII. f. 10 ; PL X. f. r >); other anal 

 plates are subsequently added, but the original large anal plate frequently 

 retains its prominence in full-grown specimens (PL VIII. f. 8 ; PL VIII. f. 

 24'), while in those genera in which the anal system is covered by a limited 

 number of anal plates, as in Arbaciadae (PL V.). Parasalenia (PL III d . f. .'). 

 Trigonocidaris (PL IV. f. 1), the number does not vary with age. 



Judging from the study of the recent species alone, the contrast between 

 an anal system connected with the apical system, as in the Desmosticha, and 

 the independent anal and genital systems of the Clypeastroids and Petalo- 

 sticha seems very striking. There are some fossil genera, Pygurus and 

 Galeropygus, which, when better known, may show how the anal opening 

 becomes gradually disconnected from the genital ring, as in both these 

 genera the anal plates must have been in contact exteriorly with the genital 

 ring. The anal system is less prominent in the Clypeastroids than in either 

 of the other suborders, and is usually reduced to a small insignificant opening- 

 covered b} 7 small plates, the position of which is extremely variable (see PL 

 XI. f. 17; PL XI". f. 2; PL XP.f. 1.4; PL XII". f. 4 ; PI- XIP. f. < ; ; 

 PL XIP. f. 4 ; PL XIII". ./'. 2, r, ; PL XIIP. f. 2, 7 ; PL XIII r . f. 2). In 

 the Petalosticha the anal system is usually large, very prominent ; the ar- 

 rangement of the plates furnishing excellent characters for discriminating 

 * A. Agassiz, Embryology of Echinoderms, Mem. Am. Acad., 1864. 



