^6 TH. MORTENSEN, (Schwed. Südpolar-Exp. 



lacral plates; in the larger ambulacral plates, I. a, II. a, III. b etc., the inner pore 

 is still not quite surrounded by the calcareous network. The sternum is already 

 distinctly amphisternous, though, as might be expected, still of a primitive type. The 

 spines have begun to appear, one especially is to be noticed on the outer end of 

 each primary interambulacral plate, as also a small one on the oral end of each of 

 them. — While the actinal skeleton is thus easily enough explained, the explanation 

 of the abactinal skeleton causes very great difficulties. As seen on PI. XVIII. Fig. 4 

 it still consists of a number of irregularly arranged plates of very different size, 

 most of them quite small. Only of three of these plates can it be said with certainty 

 which plates of the apical system they represent. The largest of them, on the right 

 anterior side, is the madreporite; the one to the right outside this, lying over the 

 terminal sucker, is evidently the right anterior ocular plate, and the larger plate to 

 the left of the anterior end of the madreporite is the anterior ocular plate (HI). 

 Spines have not yet appeared on the abactinal side. 



The ne.xt stage to be mentioned is that in which the fasciole has just made its 

 appearance, the embryo measuring now 1.8 — 1.9 mm. in length, 1.6 mm. in breadth. 

 The actinal skeleton differs from that in the foregoing stage only by the ambulacral 

 plates having increased in size adorally so as to reach the inner edge of the primary 

 interambulacral plates, the edge of the peristome being thus regular, nearly round; 

 the inner, ambulacral pore of the plates I. a, II. a, III. b etc. is now quite enclosed 

 by the plate. The abactinal skeleton, on the other hand, has undergone very im- 

 portant changes (PI. XVII. Fig. 9). The anterior ambulacrum has already 3 pores 

 and tubefeet in one series, 2 in the other, while the paired ambulacra have no tube- 

 feet developed as yet, except the primary tentacle. The primary fasciole has ap- 

 peared, as stated above, and inside it a pair of primary tubercles (spines) are seen 

 in each interambulacrum, not larger than those outside the fasciole. Of much greater 

 importance, however, are the changes seen in the apical system. The madreporite, 

 representing the right anterior genital plate (2), is very large, but with only one 

 pore; it joins the anal area, thus separating the two sides of the apical system, which 

 is accordingly ethmolytic from the beginning. In the anal area a circle of small 

 plates have appeared, and behind, in the median line, is seen a larger plate, which 

 must evidently be identified as llie posterior genital plate (5). It was found distinct 

 in all specimens of a corresponding size examined, and it must accordingly be re- 

 garded as a constant feature, and one of the highest morphological value, meaning 

 that the apical system is here typically endocyclic. There is no central (suranal) 

 plate seen in this, or, indeed, in any of the stages e.xamined; it seems then evident 

 that this plate does not occur here at all. — Probably this will hold good for all 

 the »Irregularia», as it appears to do also for the Diadematids, Echinothurids and 

 Cidarids, and likewise for the Palœechinoids. The central plate thus seems to have 



