ASTHENOSOMA IJIMAI 173 



Asthenosoma Ijimai Yosh. 



Asthenosoma Ijimai Yoshiwara, 1897. Ann. Zool. Japon., I, p. 8; PI. II, figs. 8-12. 

 Asthenosoma Ijimai Mortensen, 1904. Ann. Mag. Nat. Hist. (7) XIV, p. 87 ; Pis. Ill ; 

 V, figs. 1-3, 10, 12-14. 



Sagami Bay, Japan ; 50-55 fathoms. 



The arrangement of the tubercles in the interambulacral plates of the 

 abactinal side of the test in this species resembles closely that of A. varium. 

 The columns of primary tubercles of the actinal side do not reach beyond 

 the ambitus, and there is nothing of the characteristic arrangement of the 

 extension of the primaries toward the abactinal system so striking in many 

 species of Araeosoma. 



Mortensen criticises Yoshiwara's description of the madreporic plate, as 

 he does not consider the madreporic plate divided, but rather that the 

 madreporic pores have spread into adjoining plates. In the specimens 

 before us there seems to be no room to doubt that the madreporic plate is 

 divided, exactly as Yoshiwara described it, " into four separate pieces of 

 unequal size, the largest occupying the normal position." In addition 

 there are a dozen or more very small fragments of the plate, around the 

 genital opening, distal to the main plate. The madreporic pores spread 

 into the ocular plates on each side, but chiefly into the one in the right 

 anterior ambulacrum. The splitting up of the genital plates is now so 

 well known in the Echinothuridse that the condition of the madreporic 

 plate in this species, while interesting, is far from exceptional. On the 

 other hand, the extension of madreporic pores into anal plates is a very 

 rare phenomenon, never occurring normally so far as we know, so that 

 we cannot assent to Mortensen's interpretation of the condition in Ijimai. 

 When he says that the spread of madreporic pores " over the neighboring 

 plates " is a feature " upon the whole not very seldom occurring among 

 Echinids," it must be assumed that he means by "neighboring" plates 

 only genitals and oculars, for the presence of madreporic pores in plates 

 either inside or outside of the genito-ocular ring is most unusual. In 

 Ijimai, the plates in question must be either anal plates or parts of the 

 genital, and we feel no doubt that they are the latter. 



