DEEMATODIADEMA HORRIDUM. 



69 



In a young specimen of D. horridum of 7.3 mm. the anterior pores of the 

 left and of the right posterior ambulacra, the posterior pore of the right 

 anterior and the left anterior ambulacra slant obliquely toward the actino- 

 stome (PI. 25, fig. l). In this stage the indentations of the gills are symmet- 

 rical, but with advancing age the gill slits become asymmetrical (PI. 25, 

 fig. 3) from the extension of one of the poriferous zones (PI. 26, figs. 7, ^), 

 and in a specimen of D. ghhulosum of 30 mm., Fig. 91, the ambulacral 

 plates have been pushed towards the actinal system so as to distort and 

 crowd the poriferous plates into a long tongue (PI. 26, fig. 2), of which the 

 outer plates are ready to pass over on to the actinal membrane. 



Spheridia occur in young specimens of 9 mm. (PI. 26, fig. 3) on the 

 sixth and eighth ambulacral plates, Fig. 107. In other Echiuids they 

 usually make their first appearance on the first ambulacral plates. The 



Futs. 110-112. 



10 mm. 



Fig. ill. 



DeRMATo DIADEM A HORRIDUM 



spheridia extend the whole length of the ambulacrum. In the young 

 specimen mentioned above there are three in one zone and two in the 

 other, separated by two plates. In an older specimen of 19 mm., Figs. 108, 

 109, the spheridia are also separated by two ambulacral plates (PI. 26, 

 fig. 4), there being six and seven in each ambulacral zone. It is interesting 

 to notice that in Dermatodiadema the spheridia point toward the actino- 

 stome and not toward the abactinal pole as is the case in other Echinoids 

 where spheridia have been observed. 



The splitting of one of the anal plates of D. gbbulosum (PI. 28, fig. 3) in 

 a specimen of 25 mm.. Fig. 93, seems to indicate the manner in which 

 eight or more plates may arise from the five primitive anal plates such as 

 are found in a specimen of D. horridum of 6.8 mm. (PI. 27, fig. 2), Fig. 110. 

 In a somewhat older specimen of 10 mm., Fig. Ill, there are eight large 

 anal plates (PL 27, fig. 3), and the anal opening is covered by six oy seven 



