-^ THE VOYAGE OF H.M.S. CHALLENGER. 



thing ot the same structure which ends in some of the Clj-peastmids in the genitd 

 opeiSngs being placed outside the genital plates. j , ,i,„ 



We find tfai it is in the older groups, such as the Cidarida, and Sa emda,, that the 

 ocular plates attain their greatest development, less marked, as Wn has P"'>t«l »» ■ 

 among the Spatangoids and still less conspicuous in that most recently devebped of all 

 Minoid types, the Clypeastroids. As far as the genital plates are concerned they are by 

 no means lays connected with the genital pores, .s the oeu ar pore rs — X »- 

 nected with its plate. The genital opening has no such morphologrcal -g-^J'' '» "^ 

 relation to the test as the ocular pore, and is m the first place not ^e-lop <1 - y°-8 

 Eehmids; and furthermore these plates form a part of the coronal system at that time 

 Tn the penhrgs of the genital system may be placed in the interambulaeral area enf^ely 

 ontlide of the inital ring, a, is the case among many of the Clyi>eastroKls, and even, as I 

 ZTeLZ in the vivip:;ous Cidarid», the greater part of the genital openings are no 

 p^ed in the genital ring, thus clearly showing the want of connectron between the 

 ^ caUed .enita? plates anS the external opening of the genital organs m genera m which 

 TheTe plates attain their greatest development and in which they retain the. normal 



shape even in the adult. ^ i ^„ PlotA« XTT " 



The young specimens of PhorT^iosoma and Asthenosoma, figured on Fiates Ail. 

 XVIII XVIII.^ show some interesting points in the development of the abactma 

 tstem'of the poriferous zone, of the actinal surface, and of the structure of the coronal 

 ;C which throw considerable light on the afiinities of the EeWt7d. to other 

 group of the Echinoidea. I have already in the Revision of the Ech.m (pl- -^^^^^ 

 Lured a very smaU Asthenosoma hystrix (3-1 mm. in diameter), and caUed attention to 

 the^dSnentary separation between the plates of the ambulaeral and mterambulacra 

 tas to the absenc'e of a regular abactinal system, and also of an ^^^^^^^ 

 membrane. In the smallest specimen of Phormosoma collected by the ChaU ng^ 

 (Phormosoma uranus) (PL XVIII.^ fig. 12), measuring 8 mm. m diameter, the actma 

 membrane was already covered with plates, but plates differing m their arrangement 

 from those of the adult, and presenting features which associate these young stages iar 

 more with the Diadematid^ proper than with the Echinothurida;. , ^ o1 



The actinal membrane has, as in Centrostephanus and Echinothn., ten large buccal 

 plates, forming a nearly closed ring round the actinostome, with an outer ^-^SfJ^'^^ 

 plates ten ambulaeral perforated for tentacles, and ten interambulaeral P^- -^ This 

 Le structure is stiU seen in a young specimen o^ Phormosoma ^-^ (P]^™;^ 

 fi^ 7) measuring no less than 49 mm. in diameter, while m a specimen oi Asthenosoma 

 aracile ? (PI XII.^ fig. 6), measuring 24 mm. in diameter, the imbricatmg plates are 

 Teady well developed, but have not as yet united with the adjoining interambulaeral 

 Ites as they do in the adult. The same is the case in a young Phonnosom.. harden um, 

 tZZT^e mm. in diameter (PI. XVIII.^ fig. 4). In this stage the young Echmo- 



