ECHINOIDEA. II. 



constantly being larger than the inner one in each pair. The number of plates increases considerably; 

 whereas at 2 mm length only 2 3 pairs of plates are developed between the fascicle and the ocular 

 plate, there are 17 pairs in a specimen of n mm length. The fascicle here keeps its original position, 

 close to the anterior edge. As said above it was very difficult to trace the exact number of plates 

 of the abactinal paired ambulacra in the smaller specimens. From a size of ca. 4 mm length there was 

 no difficulty in tracing the exact number and shape of these plates; while therefore the figures i, 2 

 and 5. PI. XIII do not claim to be quite exact in this respect, the figures of the later stages give 

 them correctly. In the younger stages no pores at all are developed in these plates; at a size of 4'5 mm 

 (PI. XIII. Fig. 8) I find the pores very faintly indicated in the posterior series of the antero-lateral am- 

 bulacra. In specimens of 5'5 mm and 6'5 mm they are distinctly developed in both series of these ambu- 

 lacra (PI. XIII. Figs. 10, 12). At a size of 7-5 I find the pores of the posterior series of plates double, 

 while those of the anterior series are still simple a very interesting stage, which is kept for life 

 by the genus Agassizia. In specimens of 9 mm the pores are double in both series, though the pores as 

 well as the plates of the posterior series are still considerably larger. The antero-lateral ambulacra 

 have thus attained the petaloid condition, and their further development consists only in the enlarging 

 of the plates and pores and the gradual deepening (already at ca. 6 mm the deepening is rather dis- 

 tinctly seen), besides, of course, the adding of new plates at their upper end. The development of 

 the posterior petals begins somewhat later, on account of the original position of the transverse fasc- 

 iole close behind the apical system. In a specimen of 5'5 mm (PI. XIII. Fig. 10) I find the first plates to 

 have appeared within the fascicle; in a specimen of 6'6 rani a single pore has already appeared, and in 

 the next stage (PI. XIII. Fig. 13), 7'5' nm , three pairs of plates have developed between the fasciole and 

 the ocular plates, each with a single pore. In a specimen of 9 mm length (PI. XIII. Fig. 14) four pairs 

 of plates have developed; they are already a little widened and deepened, and the pores are double, 

 the petaloid condition thus being reached. In the plates between the transverse and the latero-anal 

 fasciole no pores are seen, but each has a rather large tubercle. . 



In the apical system also important changes take place. In the youngest specimens only two 

 large genital plates are present, viz. the two anterior ones, the right one with a single madreporic 

 pore. All the ocular plates are developed, though only that of the anterior ambulacrum is quite distinct. 

 It is an important fact that the ocular plates of the posterior paired ambulacra are separated from 

 the first beginning, at first by the anal area and later on by the two anterior genital plates; the apical 

 system thus is ethmolytic from the beginning, not passing through an ethmophract stage, as might 

 perhaps be expected from a phylogenetic point of view. The same is shown by Loven (On Pour- 

 talesia. PI. XVII) to be the case in Echinocardium flavescens, whereas the young stages examined by 

 Loven (and myself) of Spatangus purpurens and Brissopsis lyrifera are not young enough for proving 

 the non-existence of an earlier ethmophract stage in these species. The posterior genital plates 

 cannot be discerned with full certainty, till the specimens have reached a length of ca. 7 mm (PI. XIII. 

 Fig. 13). The genital pores appear at a size of 9 n mm . The madreporic pores begin to increase in 

 number in specimens of ca. 6' nm , but still at a size of 10 n mm , when the genital pores are already 

 developed, the madreporic plate has not begun to develop into that large size, which it obtains in 

 grown up specimens. 



The Ingolf-Eipedition. IV. 2. 15 



