J? or a long time the development of the Echinoderms has formed the 

 subject of a series of splendid researches. Nevertheless, there are great 

 gaps to be filled up, and the following pages may serve as a contribu- 

 tion to extend in some more or less important points our present know- 

 ledge of the embryology of this very interesting group of animals. 



Johann Müller, by whom the larval forms of many Echinoderm 

 types were long ago excellently described and figured, first gave his 

 attention to some larvse and 3^oung sea-urchins, taken from the surface 

 of the sea in the neighbourhood of Heligoland, Elsinore, Nice and Trieste, 

 and which he supposed to be young of Echinocyamus pusillus, the sole 

 representative of the order Clypeastroidea found in European waters. 

 This supposition has remained unconfirmed till the present time, nobody 

 having since published anything on the development of the animal in 

 question. Mj own researches will prove that Johann Müller was right 

 and that his larvse really were young forms of Echinocyamus. However, 

 he only treated a few developmental stages which he occasionally met 

 with on the surface of the sea and obtained by means of a tow-net. 



Except for the account of J. Müller and a very fragmentary 

 sketch by Nachtrieb on Mellita testudinata Klein, only a single paper, 

 as far as I know, has been published treating more fully the different 

 phases in the embryology of a Clypeastroid, viz »The Preliminary Obser- 

 vations» on the development of Echinarachnius parma (Lamk.) by Fewkes. 

 Thus, considering the incompleteness of our present knowledge of the 

 development of the Echinoderms in question, the following communi- 

 cations, though in many points themselves incomplete, may be of some 

 interest. And this for the additional reason that, as far as I know, this 

 is the first time the development of an Echinoid has been traced from 

 the artificially fertilized egg to the stage when the young sea-urchin has 



Nova Acta Reg. Soc. Sc. Ups. Ser. m. 1 



