SYNAPTA VIVIPARA. 59 



by the rupture of the epithelium at that point. While this has never been actually 

 observed, the theory is supported by a further examination of the genital gland. The 

 smaller internal cells of the germinal epithelium give rise to spermatozoa, which are 

 almost invariably to be found in the lumen of the gland, never on the outside with the 

 ova. If we trace the lumen forward we find it passes directly into the lumen of the 

 genital duct, the internal walls of the latter actually being formed by a continuation of 

 the germinal epithelium, which has become of uniform thickness and ciliated internally 

 (Figs. 36 and .37). The lumina of the two or three branches of the gland unite on 

 entering the duct (Fig. 36) and pass through the latter upwards towards the body-wall- 

 At no point is there any sign of communication between the genital duct and the space 

 external to the germinal epithelium in which the ova lie. If we follow the genital duct 

 upwards to where it comes in contact witli the body-wall, we find it does not fuse with 

 the ectoderm and open at once to the exterior, but simply lies in the connective tissue 

 with its end against the ectoderm (Fig. 38), and although there are probably openings 

 through which the spermatozoa pass out, they are so extremely small I have not been 

 able to demonstrate them satisfactorily. In nearly all the specimens examined the 

 genital duct contained large quantities of spermatozoa, but in no case was there any trace 

 of an ovum. That the openings at the end of the duct are not directly continuous with 

 openings through the body-wall is indicated by the occurrence of spermatozoa, sometimes 

 in large quantities, in the connective tissue surrounding the terminus of the duct (Fig. 

 38). From all these facts I am convinced that the ova pass into the body-cavity by a rup- 

 ture of the peritoneal epithelium, while the spermatozoa pass outward through the genital 

 duct to the exterior. In Cucumaria glacialis, the only other viviparous holothurian 

 concerning whose breeding we have any information, Mortensen ('94) .thinks the eggs 

 are laid on the bottom and taken up afterwards into the brood-sacks of the mother. But 

 it is manifestly impossible that the eggs of *S'. vivipara could get into the body-cavity in 

 any such way. The next question that arises is, how do the spermatozoa reach the ova 

 inside the body-cavity of the mother, and the answer^brings to light another interesting 

 modification of structure, adapted to the viviparous habit. Careful examination of the 

 rectum shows that through its wall there are direct channels of communication between 

 the body-cavity and the exterior through the anus. The wall of the rectum is folded 

 and ridged longitudinally, and at certain places, parts of these ridges have pushed out and 

 fused with invaginations from the surrounding coelomic wall, forming distinct tubes 

 connecting the interior of the rectum with the interior of the coelom (Figs. 

 30 and 31). No trace of valves or cilia was found in any of these tubes, but 

 the passage of water in and out could be easily regulated by the opening and 



