VIII EOHIXODERMATA— MORPHOLOGY OF SKELETON 321 



accessory plates, which continually increase, in number, the anal 

 aperture then forms, somewhat excentrically (Fig. 274). After a time 

 the central plate can no longer be distinguished from the accessory 

 plates. 



As a rule, i.e. in the greater number of eases, the basals and 

 radials attain, in Echinoidea, the following special significance : 



1. Each basal is perforated by a large pore or hole, through 

 which one of the Ave genital glands opens externally. On this 

 account the basals of the Echinoidea have long and almost universally 

 been called genital plates. 



2. Each radial is also perforated by a narrow canal, which opens 

 at its surface through a single (rarely double) aperture. In this 

 canal lies the terminal tentacle of the water vascular system, the 

 frequently pigmented end of which projects somewhat beyond the 

 aperture. Since these collections of pigment were formerly considered 

 to be eyes, the plates (radials) carrying them were called the ocular 

 plates. 



3. The fine, and usually very numerous, apertures of the water- 

 vascular system perforate one of the five basals (genital plates), which 

 becomes the madreporite (m). This is the right anterior plate. 



It must, however, be noted that (1) the genital apertures are not 

 necessarily connected with the basal (genital) plates. The latter 

 must not be regarded as terminal appendages of the genital ducts, 

 but as independent portions of the test. For (a) the basals are solid 

 when first developed, and are only perforated by the genital pores 

 after the genital ducts have completely developed ; (i) the genital 

 apertures in some Echinoidea lie outside the basals. For example, 

 among the Clypeadroida, in some species of the genera Laganum, Encope, 

 Mellita, etc., the genital pores lie outside the apical system, between 

 its- edge and the first two interradial plates which border it ; in 

 Cbjpeaster rosaceus, they lie in the five sutures between the interradial 

 plates, and are separated from the apical system by two or three pairs 

 of interambulacral plates (Fig. 277) ; and further, in another true 

 Echinoid, Goniopygus, the genital apertures lie interradially quite outside 

 the whole apical system. (2) The madreporite, through which water 

 flows into the stone canal, is not necessarily exclusively connected with 

 the right anterior basal (genital) plate. On the contrary, the neigh- 

 bouring genital plates, indeed, all the five plates, and in isolated cases, 

 even the neighbouring interradial plates of the corona may be perforated 

 by the afferent ducts of the stone canal. In Palceechinus each basal plate 

 is perforated by three pores, which are perhaps apertures of the stone 

 canal, perhaps genital apertures, or else partly the one and partly the 

 other. In no case, however, do the madreporic apertures extend to 

 the radials (ocular plates). 



In the Echinoidea the primitive character and especially the 

 radiate structure of the apical system may be more or less strongly 

 modified. The original cause of such modification is principally to 

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