162 



PANAMIC DEEP SEA ECHIN"!. 



becoming smaller towards the posterior edge. The rest of the actinal 

 membrane is covered with irregular rows of small elongate plates radiating 

 from the actinostome. 



The anal system is circular or slightly pyriform (Pis. 75, fig. 1 ; 78, fig. 4) ; 

 it is covered by five to six irregular rows of concentric and radiating small 

 polygonal plates, a few of which are somewhat larger on the outer margin 

 of the actinostome ; all the plates except those radiating from the actino- 

 stome carry small miliaries. 



A re-examination of two stages of C. Wi/villii, Figs. 233, 234, collected 

 by the '' Challenger," one of 18 mm. in length (PL 80, figs. 3, 4), the 



Inside. 18 mm. 



Fig. 233. Cystechinus Wyvili-ii. 



Inside. 56 mm. 



Fig. 234. Cystechixvs Wtvillii. 



other 56 mm. (PI. 80, figs. ^, ^), shows no great differences between it and 

 C. Loveni in the arrangement of the apical plates. The increase in the size 

 of the genital pores is marked in the larger specimen (PI. 80, figs. 5, 6)\ 

 they are barely visible in the smaller specimen (PI. 80, figs. 3, 4). With 

 increasing size the slightly elliptical actinal system of C. Wf/vUIii (PI. 80, 

 fig. 7) becomes pentagonal (PI. 80, fig. 2). The phyllodes are found on 

 a larger number of ambulacral plates (PI. 80, figs. 1, 2). The primary 

 tuberculation is limited to two or three tubercles on each coronal plate, and 

 the miliaries are more distant (PI. 80, figs. 1, J). In the larger specimen 

 tlic primary tubercles as well as the miliaries have increased in number, 

 so that, compared to the younger stage, they now cover the coronal plates 

 quite uniformly (PI. 80, figs. ,5^, 7). In these two specimens of C. Wf/rillii the 

 anal system is already longitudinally elliptical (PI. 80, figs, i, 7). In the 

 smallest it is covered with three to four concentric rows of polygonal plates 



