RAYS AND BILATERAL SYMMETRY 25 1 



nite position relative to the multiplying rays, and the task is 

 much lightened by determining in which interradius it belongs 

 from the general outline of the animal. However, careful 

 search has always been made in the other interradii for super- 

 numerary madreporites, but none have been found. The adult 

 stars give more difficulty, for in spite of the contrast between 

 the light buff of the calcareous plates and the dark color of the 

 dorsal wall, and although the madreporite is often i cm. acr'oss, 

 the papulae are frequently expanded to hide it completely. 

 Moreover, the equality in the size of the rays prevents orienta- 

 tion by general inspection and makes necessary a careful search 

 in each interradius. 



Having satisfied ourselves, from general examination of 

 many specimens, that a definite relation exists between the 

 madreporite and the newly forming rays, our next object was to 

 determine the order of development of the latter. 



The youngest star in the collection possessed six well de- 

 veloped rays and two so small as at first to escape notice. One 

 somewhat larger is shown in figs, i and 2. From this stage 

 onward there are, among the one hundred individuals studied, 

 nearly all the intervening stages up to the full-grown animal. 

 The eighteen-rayed stage is the only one not represented in the 

 series. 



We adopt Agassiz's and Ludwig's plan of numbering the 

 radii of the typical five-rayed echinoderm. According to this, 

 when the apical side is uppermost, counting from the madre- 

 porite clockwise, the rays are numbered from I to V. 



From a study of the metamorphosis Ludwig, '82, considers 

 that the star should be oriented by a line running interradially 

 between I and H and radially along IV, thus bringing the 

 madreporite in the left anterior interradius. 



Before Ludwig was consulted this orientation had been 

 adopted for Pycnopodia^ owing to the striking bilateral sym- 

 metry displayed in the young animals. This symmetry deter- 

 mined that the sixth ray is median anterior between I and \1 

 and by us has been called A. 



