94 THE VOYAGE OF H.M.S. CHALLENGER. • 



We find here no such sharp line of demarcation as exists in other regular Echinids between 

 the abactinal system (composed of the anal system and the plates forming the genital 

 ring) and the abactinal extremity of the coronal plates ; the abactinal system in nearly 

 all the groups except the Diadematidae being in the adult set in, as it were, independently 

 within the upper extremity of the coronal plates. In some of the Diadematidse we find 

 something analogous to the structure of the abactinal system in this group. In Echino- 

 thrix, Diadema and Astropyga the long-pointed triangular genital plates, encroaching far 

 into the median interambulacral line and separating the opposing abactinal plates of the 

 interambulacral areas, form the beginning of a closer connection between the outer plates 

 of the anal system and the new plates of the interambulacral area. This connection is so 

 close that it is well-nigh impossible to state with certainty whether the new interambu- 

 lacral plates formed at the abactinal extremity are derived from the splitting up of the 

 upper interambulacral plates or of the outer plates of the anal system (see PI. XVIII.'' 

 figs. 1, 2). In both these figures the anal plates intercalated between the genital and 

 ocular plates cannot be distinguished from the adjoining interambulacral plates. This 

 intercalation is not so apparent in the younger stage figured on Plate XVIII. fig. 9. 



In the same way at the junction of the actinal plates with those of the ambulacral 

 system the continuity is unbroken (PI. XVIII.'' fig. 3 ; PL XII. fig. 4), and although there 

 are a few additional plates formed at the line of junction of the two systems, yet at a very 

 early stage the number of plates characteristic of the actinal system are formed, and the 

 growth of the test merely separates the suckers piercing the actinal membrane ; although 

 the additional plates of the actinal membrane are formed, as I have shown, from the sepa- 

 ration of the last small intercalated ambulacral plate and its suljsequent lateral growth. 



Neither in Asthenosoma (PI. XVII. fig. 1) nor in Phormosoma (PL XII. figs. 3, 4; 

 PL XVIII. fig. 8 ; PL XVIII.'' fig. 3) are there any prominent actinal cuts for the passage 

 of the gills ; there are slight indentations between the plates at the actinal junction 

 of the ambulacral and interambulacral systems, and the gills appear in the youngest 

 stages I have examined. 



The plates covering the actinal membrane are, with the exception of a few in the 

 outer central part opposite the median interambulacral line, all ambulacral plates occujDy- 

 ing the whole of the membrane (PL XVII. fig. 4 ; PL XVIII.'' fig. 3). The arrangement 

 of the actinal plates in this family, and their close structural connection with the plates 

 of the ambulacral system, are a further step in showing the gradual modifications which 

 the actinal ambulacral plates have undergone, so as to form gradually an imbricating 

 actinal meml^rane such as is characteristic of the Cidarid^, passing into an actinal 

 membrane in which we find, as in the majority of the regular Echinids, only ten buccal 

 plates, the rest of the membrane being more or less strengthened by irregularly arranged 

 imbricating plates which may form a thick close pavement, as in some species of 

 Echinus and the like, or else an entirely bare membrane with a few scattered calcareous 



