BIHANG TILL K. SV. VET.-AKAD. HANDL. BAND 18. AFD. IV. NIO 1. 17 



development. There is a resembling discoidal or lenticular 

 form of tlie body, the same five primordial, locomotive suckers, 

 solitär}^ and provisional. In botli, tlie interiör communicates 

 with the surrounding medium through a single large water- 

 pore. In either of tliem the alimentary canal is closed at 

 Ijoth ends, astomous and aproctic; tliey are endotrophic, take 

 no food from without, and represent in some sort, as has long 

 since been recognized, ' a pupal or rather n^anphal condition 

 intermediate between the pluteus and the imago. For this 

 their definitive mode of existence they are both jjreparing by 

 tlie ultimate formation of a prehensile and masticator^' ap- 

 paratus, the completioii of which, together with that of the 

 excretory opening, will be the sign of its beginning, and b}- 

 the Imihling up witliin the envelope of a new endo-skeleton. 

 Two systems combine in its construction: the ealveinal s^'stem 

 originating round the dorsal centre, of definite constituents; 

 and the eoronal system arising from around the ventral centre, 

 ehangeable, growing upwards, itself a eombination of two 

 heterogeneous sets of plates: the ambulaera, the hrst to ap- 

 pear, binary from the beginning, heterotropic ; and the inter- 

 radia, secondary, perisomatic, commencing singly, theu beco- 

 ming binary; both sets tending, by an ancordant råte of growth. 

 to raise the test into the globular form. 



In all the Echinoidea the growth of the corona is eifected 

 by new plates being successivel}' added at the aboral termina- 

 tion of the ambulaera and the interradia, and by their in- 

 ereasing in size and solidity. As long as the animal lives, there 

 is at work, more or less, in every part of its frame a conti- 

 nuous movement of reabsorption and renewal, of taking on 

 one form and losing it for another, all in accordance with the 

 morphological canon of the species; and this process is perhaps 

 nowhere more conspicuous than in the corona of the Cidaridct 

 and Echinidfe. 



When we left the young of the Groniocidaris canaliculata 

 Al. Ag., Fl. II, fig. 7 — 9, which liere serves for a type of the 

 Cidaridffi, the primordial ambulacral plates of the corona formed 

 by themselves a slightly movable cirele round the mouth. A 



' Graber, Die Insekten, II, p. 524 — 56t. — Balfour, Comparative Em- 

 hryology, I, p. H;V2 (1880). 



