EGHLNODERMA TA —GENITAL ORGANS 



499 



lie in the apical region of the body cavity, in the interambulacra. The 

 five genital ducts ascend towards the apex, there perforate a coelomic 

 circular sinus which surrounds the rectum, pass through the genital 

 pores of the basals, and then open outward, sometimes at the tips of 

 projecting papilla?. 



The gonads. — These, in a mature condition, are large acinose 

 organs, which are suspended to the inner wall of the test by an 

 exactly interradial principal 

 suspensor, and by various 

 other bands of connective 

 tissue. They are not sur- 

 rounded by a g-enital sinus. 



The number of gonads 

 was originally five. Five 

 are found in all the regular 

 Echinoids (Cidaroida and 

 Diadematoida), and also in 

 many Clypeastroida. In the 

 Spatangoida, the Holedi/poida, 

 and many Clypeastroida, the 

 number is reduced, the pos- „ „,,, _ . .. „„•„„« a„ a»„v.,i r.r.^+in« 



' i Fig. 39i.—Cystechinus vesica A. Ag. Apical portion 



terior unpaired gonad AVlth of the test from within, with the three gonads. 1, An- 



the cenital pore belon^'ino' terior ambulacrum; 2, left anterior; 3, left posterior; 



..®, . K a . i. °T* 4, right posterior gonad; 5, circular sinus (after A. 



to it being the nrst to dis- ^gassiz). 



appear. In the Spatangoida, 



the reduction may go still further, the right anterior, and in a few 



cases the left anterior as well, disappearing (Fig. 394). 



Further details on this point are to be found in the section on the skeletal S3^steni 

 {cf. pp. 321-324 on the genital pores). It is there shown that these pores are by no 

 means necessarily limited to the basals. 



The genital apertures. — The genital papillis, on the tips of which the genital 

 apertures lie, are specially \tc11 developed in the Spatangoida. 



The ring sinus encircles tlie anus with the periproctal sinuses, the stone canal, 

 and the axial sinus. Its wall is formed on the one side by the test, and on the 

 other by a circular lamella of connective tissue which is covered on both surfaces hy 

 endothelium, on the apical surface by that of the ring sinus, and on the oral by that 

 of the general body cavity. 



The lower wall of the apical ring sinus is broken through in Dorocidaris, so 

 that the circular sinus is here in open communication with the general bodj' 

 cavity. 



In all other cases, the ring sinus is entirely closed on all sides in adult 

 Echinoids. 



In adults, there is no trace of a ring-like strand enclosed in the 

 ring sinus. The connection between the axial organ and the gonads 

 is thus lost. 



