BIHANG TILL K. SV. VBT.-AKAB. HANDL. BAND 18. AFD. IV. N:o 1. 15 



there disappeared, the substance of the disk is tliimiing oft' 

 towards the ex+ended and roiiiided margin; while a correspon- 

 ding erosion is going on at the margins of the costals 1 and T). 

 By this process the dorso-central, owing to the growth of the 

 costals 2, 3, 4, and the marginal absorption of the 1 and 5, is 

 moved towards the right side of the animal, all in accordance 

 with wliat maintains in the whole group of the Echinidte, though 

 scarcely manifest in the DiadematidjiB. Between the dorso- 

 central and these costals 1 and 5, a very narrow clear space 

 is perceived, forming nearly a semicircle, the first indication 

 of the future periproct, but there is no trace in it of an ossicle. 

 as in the former specimen. The animal here described is evi- 

 dently younger. It assuredly is not imaginary to assnme that. 

 if it had been caught sooner, may be by some few days only. 

 the narrow opening had not existed and the dorso-central os- 

 sicle would have presented its priniordial, V)nt transitory an- 

 cestral, pentagonal form. 



The costal 2, somewhat broader than the others, has stil] 

 the solitary large water-pore. The radials are broad in rela- 

 tion to their length, and slightly concave at the free margin. 

 Över nearly the whole of the calycinal system the epistroma 

 is thinly expanding, more distinctly at the borders, as a glo- 

 meration of minute globules. On the dorso-central, apparently 

 the last formed of the constituents and already on the way to 

 dissolve, it is hardly to be perceived. 



Another young specimen, fig. 28, not a little larger, 2,-> 

 mm. in diameter, apparently of the same species as the fore- 

 going, has the mouth free, the teeth naked, the gills formed, 

 and the solitary interradial greatly reduced, but the hetero- 

 tropic absence of the pedicellar pores I a — V h is still obvions 

 in the priniordial ambulacrals which keep their places near 

 the future peristome. 



This prominent feature in the young animal, the retarded 

 appearance of the other of the two heterotropic series of pri- 

 mar}^ pedicels, the I a — V ?>, seems however not to be of 

 constant occurrence in the different genera, or perhaps more 

 transient in some forms than in others. It has already been 

 seen that the young Echinus, woodcut p. 11, which has just 

 completed its pluteal stage, and has acquired only one ambu- 

 lacral circle, presents a pore and a pedicel in every one of 

 its plates. 



