ECHINOCYAMUS PUSILLUS. 47 



papillata, which he believes to be abnormal. However, at present we 

 may take it for granted, that the unpaired rod characterizes the larvée 

 of the Spatangoids. With regard to the Clypeastroids, they appear to 

 have the Ptuteus more rounded posteriorly, to be devoid of epaulettes 

 and the terminal arm, and to be in possession of but eight arms. 



IX. 

 The development of the young sea-urchin. 



The changes which take place in the larvée of the Echinoids 

 during the metamorphosis are indeed of the highest interest, and, con- 

 sidering the incompleteness of our present knowledge, they certainly 

 deserve a careful reexamination. Notwithstanding this, I am compelled 

 for several reasons to treat the chapter in question in a very summary 

 manner. Firstly, I could not obtain larvae in an advanced state before 

 the beginning of August, and on the first of September I was obliged to 

 resume my teaching at the University of Upsala, and consequently there 

 was not time enough at my disposal to bring my studies to the desi- 

 red end. Moreover from want of time I have been unable to prosecute 

 my researches at Upsala. Lastly, in order to present a true account of 

 the young sea-urchins at my disposal, I am greatlj^ in need of specimens 

 in somewhat older conditions. More than once therefore I have found 

 myself inclined to put off the publishing of this chapter till the future, 

 but being in uncertainty as to the time when I may get the chance of 

 renewing my studies on the development of Echinocyamus, I have de- 

 cided to send forth my researches even in their incompleteness. Thus, 

 the following pages only pretend to be considered as a preliminary ac- 

 count or rather as an explanation of the plates. 



As has been previously noticed, the vaso-peritoneal vesicle becomes 

 constricted ofip from the archenteron in a very early stage of the deve- 

 lopment and becomes rapidly separated into two vesicles, which arrange 

 themselves on each side of the oesophagus. The constriction of the ar- 

 chenteron and the separation of the vesicles seem to pass almost simul- 

 taneously. In a short time they are also spread backwards over a part 



