76 THE VOYAGE OF H.M.S. CHALLENGER. 



1, 2, 5, 12) and Asthenosoma show but the shghest possible trace of the lapping of the 

 edge of the plates of the ambulacral and interambulacral areas. 



It is only in somewhat older stages (PI. XII." figs. 4, 5 ; Pis. XVIII., XVIII.= fig.^. 

 3, 6, 7) that the lapping of the sutures becomes apparent, or that the interstices between 

 the plates are formed ; unfortunately there are not among the Challenger specimens 

 enough small specimens to enable me to open a sufficient number of stages to examine 

 the changes in the imbrication of the plates due to growth. 



There are, however, among the Echinids of the last " Blake " expedition a large 

 number of young stages both of Phormosoma and Asthenosoma, and I hope to return 

 to this subject in my final report on the " Blake " Echinoidea. 



On the abactinal surface the arrangement of the tubercles characteristic of the adult 

 seems to be developed at a comparatively early stage (see young of Asthenosoma peUucidurn, 

 PL XVIII. of Phormosoma tenue, and Phormosoma luculentum, PI. XVIII.''), and the 

 same is the case with the characteristic features in the great development of the primary 

 tubercles of the actinal surface (see PL XVIII. figs. 1, 4, 8, and PL XVIII." figs. 4, 7). 



The irregularity of the course of the poriferous zone on the actinal surface is not 

 characteristic of the adult alone. In the earliest stages of Phormosoma, which I have 

 thus far examined (PL XVIII.'^ figs. 4, 7 ; PL XII. " figs. 3, 6, 8), the same iiTegularity 

 already exists, the poriferous zone extending in a sti'aggling line of pores from the 

 ambitus to the actinal membrane. The poriferous zone of the abactinal surface in the 

 young stages is also narrower than in the adult and is confined mainly to the outer edge 

 of the ambulacral area, extending in a more or less irregular undulating zigzag line of pairs 

 of pores from the ambitus to the abactinal system (PL XII." figs. 4, 5, 9 ; PL XVIII." fig. 

 6) , and the pores attain only in later stages the characteristic arrangement of the adult. 



In the young Phormosoma the small intercalated plate of the poriferous zone is 

 not placed as it is in the adult about halfway between the outer edge of the ambulacral 

 zone and the median line, but is found close to the outer edge, gradually passing with 

 increasing age and size towards its more central position (Pis. XVII. and XVIIL). 



Although in one of the species {Phormosoma luculentum, PL XVIII."^ fig. 3) the definite 

 arrangement of the pores characteristic of that species was already perceptible in a 

 specimen measuring not more than 36 mm. in diameter, the alternate arrangement of the 

 poriferous zone seems to be attained earlier in Phormosoma than in Asthenosoma (see PL 

 XVIIL) ; though in a young Asthenosoma {A. gracilef), measuring 30 mm. in diameter, 

 the poriferous zone was already separated into its two components much as in the adult. 

 In aU the young Echinothuridse in which the imbricating membrane of the actinal 

 surface was partly developed, the gills are already present, though in the early stages they 

 appear as simple diminutive forks, and soon increase in size to form somewhat more fan- 

 shaped appendages, with three to four short digits (PL XII.* fig. 6, PL XVIII." fig. 4, 7) ; 

 but in the young as in the old, even when highly developed, as in Asthenosoma tessellatam, 



