EXPLANATION OF THE PLATES. 



(9; 



Fig. 2. Transverse section of the same amliulacrum, 

 farther from the margin of the disk, across the peri- 

 pheric end of the radiating folds of the lower floor. 

 lower floor ; o' ambulacral chamber ; h radiating 

 folds of the lower floor ; a? section of the c hymiferous 

 tubes. 

 Fig. 3. Transverse section of the same amliulacrum, across 

 the middle of the radiating folds of the lower floor 

 and extending to the centre of the adjoining inter- 

 ambulacrum. 0^ ambulacral chamber ; h radiating folds 

 of the ambulacrum ; c radiating folds of the adjoining 

 interambulacrum ; / tentacles of the adjoining inter- 

 ambulacrum ; a} interambulacral chamber : o' chymife- 

 rous tubes of the adjoining interambulacrum ; g thick- 

 ness of the upper floor ; o lower floor. 

 Fig. 4. Transverse section of the same ambulacrum, across 

 the region where the concentric folds of the lower 

 floor occur, o' ambulacral chamber ; a' interambulacral 

 chamber of the adjoining interambulacrum; e lower 

 floor of the ambulacrum with concentric folds ; e' lower 

 floor of the interambulacrum with concentric folds ; 

 g thickness of the upper floor. The isthmus between 

 the two corresponds to the broad radiating bands of 

 fig. 1, PI. V. 



Fig. 5. Transverse section of an interambulacrum, in the 

 region where the concentric folds occur. g upper 

 floor ; a lower floor of the interambulacrum ; o lower 

 floor of the adjoining ambulacrmn ; «' interambulacral 

 chamber ; o' ambulacral chamber of the adjoining 

 ambulacrum ; e and e' folds of the lower floor of the 

 ambulacnmi and of the interambulacrmn. 



Fig. 6. Transverse section of the same interambulacrum, 

 across the tentacles, a} interambulacral chamber ; h 

 radiating folds of the adjoining ambulacrum ; c raili- 

 ating folds of the interambulacrum ; e' lower floor of 

 the interambulacrum ; /' oj^enings of the cavities of 

 the tentacles ; / tentacles ; g g upper floor. 



Fig. 7. Longitudinal section of an ambulacrum, o eye ; 

 o' ambulacral chamber; e concentric folds of the lower 

 floor ; g upper floor. 



Fig. 8. Longitudinal section of the ocular chymiferous 

 tube. eye ; 6^ peripheric prolongation of the ambu- 

 lacral chamber or chjTniferous tube of the eye. 



Fig. 9. Transverse section of part of the upper floor, at 

 a little distance from the central circular disk, o 

 ambulacral furrow leading into an ambulacral chamber ; 

 a interambulacral furrow leading into an interambu- 

 lacral chamber. 



Fig. 10. Transverse section of part of the upper floor, 

 near the central circular (Usk. o beginning of the 



ambulacral furrow ; a n beginning of two adjoining 

 interambulacral fuiTOws. 



Fig. 11. Transverse section of a marginal lobe of the 

 disk, corresponding to the left part of fig. 1. g g 

 upper floor ; o o lower floor ; a' a' chymiferous 

 tubes. 



Fig. 12. Part of the lower floor, seen from the outer 

 surface, rf' rf" concentric folds in that part of the 

 lower floor which is detached from the upper floor ; 

 e concentric folds in that part of the lower floor which 

 is united with the upper floor along the line k\ f' 

 radiating folds intersecting the circular or concentric 

 folds ; c radiating folds of an interambulacral zone ; 

 h radiating folds of the adjoining ambulacral zone. 



Fig. 1.3. Longitudinal section of an interambulacrum, in 

 the direction of f' in fig. 12. rf' concentric folds 

 corresponding to rf" in fig. 12; e' concentric folds 

 corresponding to e' in fig. 1 2 ; /' openings of the 

 cavities of the tentacles in the prolongation of the 

 same zone ; / the tentacles. 



Fig. 14. Transverse section across the middle of a Cya- 

 uea, to show the general relations of the upper and 

 lower floors of the disk. The section passes through 

 two interambulacral, and through two sexual pouches, 

 and divides the mouth so as to leave two oral lobes 

 entire. This figure is much i-educed. g g upper 

 floor ; /' p- and e e lower floor ; /' /' being the 

 openings of the tentacles leading into the ambulacral 

 chamber, and e e the concentric folds of the lower 

 floor ; // tentacles ; os, os, sexual pouches ; 3 3 thick- 

 ened ring of the mouth; d d the oral lobes; d* d* 

 the marginal folds of the oral lobes. 



Fin. 1-5 is intended to show the connection of the oral 

 lobes with the lower floor, e e' and d} being the part 

 of the lower floor with concentric folds. 1 is one 

 nf the pillars arising with two roots from the margin 

 of rf' to form one of the corners of the mouth, while, 

 at the same time, supporting the lateral walls of the 

 main cavity, 5 mai'king the point where the pillar 

 divides again to form the two halves of each oral 

 lobe, as seen in PI. 4, fig. 1 .5, 2 being one of the 

 branches ; 3 is the thickened ring of the mouth con- 

 necting the four pillars; under its thickest part. 4. 

 the oral lobes bend inwanl to shut the mouth ; on 

 folds of a sexual pouch, o o being the sexual organs ; 

 d d oral lobes; d* rf* folds of the oral lobes. 

 Fig. 16. Internal view of the mouth, the four pillars 

 supporting its four corners and their prolongations into 

 the oral lobes being cut through in different ways, so 

 as to exhibit in different sections the varying thick- 



