ORAL AND APICAL SYSTEMS OF THE ECfllNODERMS. 357 



pentagonal shape, but as maturity is approached it generally 

 undergoes very considerable modification, being gradually 

 resorbed from one side, simultaneously with the develop- 

 ment of the anal opening. This circumstance seems to have 

 led Agassiz^ to name it the suhanal plate. 



As the test enlarges, other plates are gradually added to 

 the anal system, and the primitive central disc becomes much 

 less conspicuous. But in Salenia it is never resorbed to such 

 a considerable extent as in other Echini, and retains its 

 original preponderance and pentagonal form throughout 

 life, filling up the central space of the apical system (fig. 

 II, 1). In one species, S. goesiana, Loven, no secondary 

 anal plates are developed, and even when they do appear, as 

 in tlie ordinary SalenidcE, they remain very small throughout 

 life. 



Fig. II. — Apical system of Salenia (after Loveu). A. Anus. 1. 

 Central disc or subanal plate. 3. Genital plates=costals (Loven). 

 4. Ocular plates=radials (Loven). 



In either case, however, the anal opening is excentric, 

 trenching on one of the posterolateral angles of the partially 

 resorbed central disc (fig. ii, a) ; but in the Cidarida the 

 anal opening in the adult animal is placed in the central 

 part of the apical system, and, as in most Saletiidce, is sur- 

 rounded by the remains of the central disc together with 

 numerous secondary anal plates. 



In young Urchins, and in Salenia throughout life, the 

 whole of the apical system is marked externally by a pecu- 

 liar striation. This is also found on the calyx of the curious 

 fossil Crinoid Marsupites, which seems to agree so very 

 closely with the primitive condition of the apical system in 

 the Echinoidea that Loven- has been led to institute a com- 

 parison between the two. Thus, the large pentagonal plate 



1 ' Loven, Etudes, &c.,' p. 70, figs. 170—170. 



2 Loc, cit., pp. 71, 72. 



