492 COMPARATIVE ANATOMY chap. 



far forward in the dorsal mesentery, to pass through the body wall 

 at some point of the anterior half of the body in the dorsal median 

 line, and to open outward through the usually single genital aperture. 



The distance of tliis aperture from tlie extreme anterior end of the body, how- 

 ever, varies very greatly. It is greatest in the Elasipoda, and becomes smaller in 

 the Aspidochirotcv. In the MoliMdiidce and Synapfidce the genital aperture lies 

 immediately behind tiie circle of tentacles, and in the DendrochirotcB it shifts into 

 that circle, even reaching its inner side. It is found behind the middle of the body 

 only in Psychropotes longicauda. 



The genital aperture is usually inconspicuous. Occasionally it is found on the 

 tip of a genital papilla ; in species of the genera.' Thyone and Cucumaria, this is the 

 case only in males, a slight sexual dimorphism thus arising. 



The occurrence of several genital apertures (2, 4, 8, 16 in certain Elasipoda) is 

 quite exceptional. They always lielong to one and the same gonad, and to one 

 genital duct. This latter in such cases, before emerging, divides dichotomously into 

 as many branches as there are apertures. 



C. Asteroidea (Fig. 385, p. 484). 



The genital organs are here developed as five pairs of bundles of 

 gonadial tubes, or as five pairs of rows of such bundles. These pro- 

 ject freely into the body cavity ; their bases are attached to the apical 

 (dorsal) body wall, generally at the aj^ical edge of the supramarginal 

 plates, or on a level with this edge. Exactly over the point of 

 attachment, i.e. over the base of each gonadial tuft, the efferent duct 

 traverses the body wall (between two neighbouring skeletal plates) to 

 open outward at the surface through one, less frequently through 

 several, genital apertures. These apertures are quite small, and are 

 often only clearly visible at the season of sexual maturity, when the 

 genital products are ejected. 



The bases of all the gonadial bundles are still connected with the 

 axial organ even in the adult {cf. p. 445 on the axial organ and the 

 axial sinus). The axial organ is continued along the inner apical 

 (dorsal) body wall (that turned to the ccelom) as a pentagonal strand 

 running round the apical pole and the anus, which agrees in 

 structure with the organ itself. At each of the five interradially 

 placed corners of the ring it sends off a pair of strands which run 

 peripherally. There are thus in all five pairs of strands radiating 

 from the ring ; these run to the bases of the five pairs of gonadial 

 tufts, and where these are in rows, from tuft to tuft of each row, 

 connecting their bases. 



Just as the axial organ is surrounded by the axial siiuis, so are 

 all its derivatives surrounded by a coelomic sinus, a direct prolonga- 

 tion of the former. 



The aboral ring-like strand lies in a ring-sinus, attached to its 

 wall by a suspensory band. This sinus is also continued along the 

 five pairs of strands which radiate from the ring-like strand ; when it 



