Prof. S. Lov^n on the Stnicture of the Echinoidea. 289 



Thus in the irregular Echinoidea tlie peristome of the triviura 

 is asymmetrical with relation to the antero-posterior axis ; of its 

 six ambulacral plates, the right side of the animal has two 

 simple and one binary, and the left side two binary and one 

 simple. The bivium, on the contrary, includes symmetrically 

 the unpaired interradium. Eight and left are determined here 

 by the position of the anal aperture, and also, except in the 

 Ch/peastridce, by that of the madreporic plate ; in the regular 

 Echinoidea, hitherto, by the latter alone. 



Of the five genital plates, the Sjmtangidce are destitute of 

 the hinder one, which elsewhere is placed near the end of the 

 impaired, anal interradium. It has never been developed, any 

 more than the genital gland, which otherwise, as in the four 

 others, should have its efferent duct through it. In all the 

 known living Spatangidce^ apparently with the exception of a 

 single genus, its place is occupied by the madreporic plate ; 

 the filtering-apparatus of the aquiferous system, which spreads 

 itself out in the posterior part of the vertex, often occupies a 

 greater space than any of the four genital plates, is posteriorly 

 in immediate contact with the last plates of the anal inter- 



Fio-. 1. 



Echinocarclknn cordatiim (Penii.), Schizastcr frtit/ilis (Diib. & Ivor.), 

 Ahatiis PhiUppii, n., Heiniaster cxperffitus, n. 



radium, by which it is enclosed, and separates from each other 

 the eye-plates of the bivium and the lateral genital plates. 

 There is no suture to form a boundary between the right 

 anterior genital plate and the madreporic area ; and Avhen the 

 latter has a great extension it is this plate that first enters into 



