4 Prof. J. Miiller on the Structure of the Echinoderms. 



right anterior genital plate towards tlie centre (Anat. Studien 

 iiber die Echinodermen, Archiv 1850). 



Among the regular Echinida the elongated forms of the 

 Echmometrce are particularly interesting. 



In Echinometra acufera the anus is approximated to one, the 

 longest radius, so that the antero-posterior diameter is the 

 longest. If the azygos middle radius of an Echinometra be 

 placed in front, the madreporic plate will constantly be found to 

 lie laterally and to be the left posterior genital plate. In this 

 position the radii and inter-radii of Echinometra are perfectly 

 symmetrical, and it is the only position in which they are so. 

 In the genera Heterocentrotus, Br. {Acrocladia, Ag.), and Colo- 

 bocentrotus^j Br. {Podophora, Ag.), which have been separated 

 from Echinometra, the body is only symmetrical when placed 

 transversely, as was observed by Brandt in the first-named genus, 

 and expressed by him in the phrase ' corpus transversum' , but over- 

 looked by Agassiz, who regards their forms as oblique, in com- 

 mon with Echinometra. The proper position of these shells is — 

 with the greatest diameter of the shell transversely, the smallest 

 longitudinally, and the smallest radius anteriorly ; thus placed, 

 the madreporic plate in Heterocentrus trigonarius, mammillatus, 

 and in Colobocsntrus atratus, is the right anterior genital plate. 

 Among twenty-four specimens of Colohocentrus atratus, twenty- 

 three had the madreporic plate thus situated ; one however had 

 this plate in the left posterior inter-radius. Such variations de- 

 pend upon whether the Echinus-disc with its madreporic canal 

 is formed on the right or on the left side of the larval frame- 

 work — deviations which, as my figures of Echinus-larvse from 

 Helgoland show, have already been observed. 



In Echinometra the transverse symmetry occurs rarely as an 

 anomaly ; among a great number of Echinometrce (about eighty 

 specimens, including all the known species) only four transversely 

 symmetrical specimens were met with. Among fifteen examples 

 of Echinometra acufera, BL, from the same locality (Venezuela), 

 there was one ; among six of E. oblonga, BL, one also was trans- 

 versely symmetrical, i. e. one had the azygos radius shortest ; 

 in the others, the azygos radius was the longest of the five. 



The division of the Echinometrce into genera rejected by Sou- 

 leyet (Bonite) is hence shown to be natural. 



In the Spatangidce of the present epoch the madreporic plate 

 is commonly situated at the posterior extremity of the apex, but 

 even in this case its position is problematical ; I have already 



* It may be preferable to shorten these names into Ueterocentrus anci 

 Colohocentrus, 



