ECHINOCYAMUS PUSILLUS. 23 



area, we almost entirely lack the information necessary for forming a 

 correct opinion. As far as our present knowledge goes, only the Bla- 

 stula of Echinus microtuberculatus seems to be provided with a thickened 

 apical area. 



Though the information which we possess concerning the Bla- 

 stula stage of Spatangoids is incomplete, we find that according to 

 Selenka ') and Fleischman ^) the larva of Echinocardium cordaùum pre- 

 sents an almost spherical or slightly oval shape and is devoid of any- 

 obvious apical area of differentiated cells. 



The incompleteness of our present knowledge prevents a closer 

 comparison, but judging from what is known we are entitled to con- 

 sider the elongate, cylindrical Blastula of Echinocyamus as rather in- 

 teresting and singular of its kind. Another peculiarity characterizing 

 the larva in question is the early formation of calcareous spicules, PL 

 II, fig. 35, which, as far as I have been able to discover from the litera- 

 ture at my disposal, never takes place in the Blastula of other Echi- 

 noids, except in a subsequent stadium. Probably the same is the case 

 with the differentiated cell-area at the animal pole, which presents itself 

 in this very early stage of the development of Echinocyamus; because, 

 to judge from literature as well as from what I have seen myself 

 during my studies at the sea, such a polar differentiation takes place in 

 many forms of Echinoid larvae but not till the gastrula stage. This 

 is confirmed by Nachtrieb ^), who has found that this is the case 

 not only in Mellita, but even in Strongylocentrotus and the Spatangoid 

 Moira atropos. 



The differentiated cell-area at the animal pole with its tuft of long 

 cilia probably serves as a larval organ of sense and therefore claims a 

 certain interest. 



1) Keimblätter and Organanlage der Echiniden. 1879. pi. V — VII. 



2) Die Entwicklung des Eies von Echinocardium cordatum. 1888. pl. XIV. 



3) Preliminary Notes on the Echinoderms of Beaufort. 1885. p. 67. 



