VIII EGHINODERMATA— GENITAL ORGANS 491 



(3) a layer of connective tissue ; and (4) the inner germinal epithelium. 

 The muscle layer is often wanting. 



According to the morphology of the genital organs the Echino- 

 derms fall into two groups. 



In the larger principal group, containing the Echiiioidea, Aste- 

 roideii, Ophiuroidea, and Crinoidea, there are several gonads, each with 

 a duct and an aperture ; they follow, in their arrangement, the radial 

 structure of the body, showing at the same time close relations to the 

 axial organ (or to the wall of the axial sinus). The axial organ has 

 been compared to a trunk, of which the gonads, as direct prolonga- 

 tions, are the fruitful branches, on which the sexual products ripen, 

 as fruit. 



The direct organic connection between the axial organ and the 

 gonads persists throughout life in the Asteroidea, Ophiuroidea, and 

 perhaps also in the Crinoidea ; in the Echinoidea it is only ontogenetic, 

 and ceases in the adult. 



The second group is formed by the Holothurioidea, in which there 

 is neither axial organ nor axial sinus. The genital organs consist of 

 a single tuft of gonadial tubes. This tuft lies in the body cavity in 

 the median (dorsal) interradius, and sends off a duct which runs 

 forward in the dorsal mesentery, and opens outward in the anterior 

 region of the body, often very near the mouth. 



There is, as a rule, no difference in the microscopic structure 

 and external appearance of the male and female genital organs in 

 Echinoderms. In some cases, however, at the time of maturity, they 

 may be distinguished by their different colouring. 



Secondary sexual characters have been noticed only in very rare 

 cases. 



B. Holothurioidea (Figs. 371 and 383, pp. 451 and 477). 



In all Holothurioidea, the gonads are developed as a single tuft of 

 genital tubes, lying in the dorsal interradius. All the tubes of the 

 tuft converge towards one point, and open into the base of the gonad, 

 which lies in the dorsal mesentery, and is often somewhat widened 

 for the reception of the sexual products. 



The gonadial tube.s are either simple or hranched ; in number and size they vary 

 ■within pretty wide limits according to the species and the stage of maturity attained. 

 They may exceed the body in length. They usually hang from the base of the 

 gonad, on the two sides of the mesentery, into the body cavity, but there are cases 

 in which they are developed only on one side — the left — of the mesentery (in 

 Holothuria, MilUerm, Labidodemas among the Aspidochirotce, and in many 

 Elpidiidce). The base of the gonad lies in the anterior half of the body, often very 

 near its anterior end (especially in Syncqitidce and Molpadiidcc, but also in many 

 Aspidochirotm and Dcndrochirohc). 



From the base of the gonad the genital duet runs more or less 



