450 RECOED OF CURRENT RESEARCHES RELATING TO 



Stewart expected tliat some observer would, an organ of similar 

 character in the Diadematidae. There can be but little doubt that 

 these organs have the functions of the gills which are found round 

 the actinostome in the rest of the regular Echinida, although Dr. 

 Ludwig feels himself unable to pronounce an opinion on the subject ; 

 in any case, his suggestion that living examples should be examined 

 with a view to settle the question, is one deserving publicity. 



Asthenosoma varium* — Dr. Hubert Ludwig gives a careful 

 account of this very interesting Echinothurid ; in dealing with its 

 affinities, he points out that it will be of considerable importance to 

 definitely settle whether these creatures have or have not the peri- 

 gtomial branchiae. Sir Wyville Thomson thought not, but Dr. 

 Ludwig's specimens do certainly possess them, and as their presence 

 is constant enough in all regular Echinida, with the exception of the 

 Cidaridfe, the writer thinks that more careful examination will reveal 

 their presence in the specimens of Professors Thomson aud Agassiz. 

 They are found in the peripheral portion of the buccal membrane, 

 between the ambulacral plates, which are continued on to the buccal 

 membrane ; but there are no notches in the edges of the interam- 

 bulacral plates for their reception, and this, together with their 

 delicacy and small size, is probably the cause of their having been 

 hitherto overlooked. 



Other points in their organization are not so markedly Cidarid as 

 has been hitherto supposed ; and this leads us to a sy>tem of classifi- 

 catioti of the regular Echinida in which the Cidaridae are marked off 

 from all the rest ; thus we have — 



I. Echinida with ambulacral and iuterambulacral plates on the 



buccal membrane. 

 Cidaridfe. 



II. Echinida with ambulacral plates only on the buccal membrane. 



A. More than the two buccal plates for each ambulacrum. 



Echinothuridae. 



B. Only two buccal plates for each ambulacrum. 



SaleniadfE, Arbaciadae, Diadematidic, Echinometradse, 

 Echinidas. 



As all observers, with the excej)tion of Alex. Agassiz, have failed 

 to detect the presence of gills on the buccal membrane of the 

 Cidaridae, and as his statement cannot be borne out by his figures, 

 and as the spaces which are taken by Agassiz in the figure of 

 Johannes Miiller (who was the first to point out the absence of 

 peristomial gills in the Cidaridae) to be gill-clefts are probably due to 

 shrinking of the membrane,| Dr. Ludwig proposes to give to group I. 

 the title of Abranchiata, and to group II. that of Brauchiata. 



As Dr. Ludwig shows, the presence of two genital pores in one of 

 the genital plates is a point of especial interest ; similar observations 

 have been made on recent forms by other observers ; Baily found in 

 Archceocidaris Harteiana six pores in all but the madrci)oric plate; 



* ' Zeitbchr. wiss. Zool.,' xxxiv. (1880) p. 70. 



t The researches of Stewart aud Ludwig coufirm Miiller '.s views (cf. the pre- 

 cediug note). 



