334 p. HERBEKT CARPENTER. 



(both having the anal interradius backwards) be "orientirt," 

 according to Ludwig's theory of their homologies, viz. the former 

 resting on its dorsal and the latter on its ventral face, the position 

 of the water-pore is south-west in the Crinoid or beyond the 

 anus to the watch-hand; but in the Starfish it is south-east or 

 before the anus to the watch-hand (fig. iv). Turn the Crinoid 

 over, however, so that it has its dorsal surface upwards like the 

 Starfish (fig. iii), and the water-pore is south-east in both cases, 

 i. e. in the right posterior interradius. 



According to Ludwig, the perforation of one of the skeletal 

 plates by the water-pore is a primary character, and fixes its 

 homologies once and for all. 



So far as this relates to the radial symmetry of the Echinoderm 

 I fully agree with him. But may not the precise position of the 

 madreporite depend upon the future relations of the plates of the 

 skeleton, and on the times at which they respectively appear? 

 Thus, in the Asterids the rudimentary genital plates are either 

 relatively large, extending right out from the dorsocentral to 

 the edge of the disc, or they are smaller, as in Asterina, and 

 develop near the edge at a little distance from the dorso- 

 central. This is in correspondence with their relations in the 

 adult, while the radials appear beyond them, to be carried 

 outwards on the growing arms. Even in Asteriiia} " Der Riick- 

 enporus liegt also urspriinglich nicht in der spiiteren Madreporen- 

 platte, sondern links von derselben;" and Ludwig admits that 

 in other Starfishes there is no primary connection between the 

 water-pore and any genital plate. In the larva figured by 

 Metschnikoff',2 which has four pairs of spines on each ray, a well- 

 developed madreporic plate is situated at the edge of the disc, 

 quite outside the circle of genital plates. 



In the young Asterias glacialis, according to Loven,^ the 

 water-pore does not appear in its genital plate till after the 

 Starfish is more than 2 mm. in diameter, and there are at least 

 three spiniferous plates between the genitals and radials. In the 

 young Asteracanthion of four months, figured by Agassiz/ which 

 has three pairs of tube-feet, the madreporic body is still on the 

 edge of the disc of the lower or actinal side ; and even in the in- 

 dividual one year old which he figures there is no trace of the 

 madreporite on the dorsal side. 



One would greatly like to know the position of the madrepo- 



1 Loc. cit., p. 50, Taf. iii, fig. 41. 



^ ' Studien iiber die Entwickelung der Echinodermen und Nemertinen,' 

 'Mem. St. Petersb. Acad.,' vii*^ serie, tome xiv, No. 3, pi. xii, fig. 1. 

 3 Loc. cit., p. 87, figs. 257, 259, 260. 

 * ' North American Starfishes,' pi. vi, fig. 10. 



