294 THE ECHINOIDEA 



gonad there aborts ; the gonads thus occur as two lateral pairs, and 

 increase the bilateral symmetry of the Echinoid. This is further 

 strengthened by changes in the structure of the apical system 

 of plates. This system, as in Cidaris or Echinus (Fig. I.), consists 

 of a double circle of plates. The inner circle is formed by the 

 five genital plates, which are often called the basals. As the 

 latter name has been given to them on the ground that they are 

 the homologues of the basal plates in Crinoids, the older name of 

 genitals is here retained. The outer circle of plates occur in the 

 angles between the genitals, and as they bear the " oculi," they 

 are called the oculars ; they have in turn been regarded as homo- 

 logous with the radial plates of the Crinoid cup and called "radials," 

 while Cue"not, on a different theory of homology, calls them the 

 "terminals." As the double circle of plates surrounds the anus, 

 this form of apical system is known as " eridocyclic." From its 

 typical arrangement, as seen in Cidaris, variation takes place in 

 two directions. The extreme of one line is seen in Tiarechinus (Fig. 

 XVIII.) and Lysechinus, in which the apical system of plates forms 

 either half or nearly the whole of the test. In the other direction the 

 plates become less important ; in Aspidocliadema (Fig. XXII.) they 

 form a single ring of ten plates ; and in Asthenqsoma (Fig. X.) they 

 are either reduced to ten rudimentary plates of no 



9 < Mjj fimctional importance, or are altogether absent. 

 wg> ^^^ The membrane which lies between the anus 



fl "%"" % an d the genital plates is generally covered by num- 

 tf^1 erous small plates, known as the "anal plates." 

 FIG. x. I n some genera, such as Acrosalenia, some 



Apical system of of the anal plates are large and thick, and are 

 the s a o ^- firmly attached to the gen itals. One of these 

 (After p. and F. Sar- plates may increase at the expense of the rest, 

 until, as in the genus Salenia, there is one large 

 suranal plate attached to the genitals (Fig. XIX.). This plate 

 has been regarded as the homologue of the imaginary dorso- 

 central plate of the Crinoids. This plate pushes the anus back- 

 ward from its originally central position. 



A tendency towards the retrogression of the anus is shown 

 in all groups of Echinoids. One of its effects is the pulling 

 out of the posterior plates of the apical system, and the consoli- 

 dation and increase of those in front. Thus in Zeuglopleurus the 

 anterior genitals meet along the middle line, while the posterior 

 genitals become narrow and are completely separated from one 

 another by the oculars. In Pygaster (Fig. XI. 1) the retrogression 

 has become so marked that the anus lies just outside the apical 

 system, which is therefore " exocyclic." In Clypeus (Fig. XI. 2, 3) 

 the anus becomes completely detached from the apical system, 

 which is no longer a ring of plates, but a compact group. 



