ECH1NODERMA GENERAL DESCRIPTION 



The Genital Organs throw light on the axial organ, since it 

 exists only in those classes in which the gonads are affected by 

 radiate symmetry, and not in the Holothurians. It follows that 

 the axial organ was a secondary development. Ontogenetically 

 the genital strands bud off from one end of it, where a ring is 



ac/ 



FIG. XX. 



The axial organ. 1, longitudinal section through part of the organ in the sea-urchin, 

 Sphaercchinus yrunularis (after Leipoldt). x 350. pg, pigment masses; lac, peripheral blood 

 lacunae of the organ ; ire, wandering cells. 2, longitudinal section, showing relations of the 

 organ to the chambered organ in young Antedon (after Perrier). ax, cells of axial organ ; /, 

 fibrous membrane, which partly envelopes it ; <;>, epithelial coat continuous with chambered 

 organ ; s, septum of chambered organ; n, nerves proceeding from libiillar envelope of same ; 

 c, cirri, one just budding out ; ac, atrophied axial canal of stem, continuous with axial organ. 



formed, and with the extensions of these go also extensions of the 

 axial sinus (Fig. XXL). The single gonad of the Holothurioidea, 

 connected with the dorsal mesentery, appears therefore to be the 

 homologue of the axial sinus and organ rather than of any one of 

 the interradial gonads of the other classes. The gonads, therefore, 



