76 
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 (III). 
Spines have not yet appeared on the abactinal side. 
The next 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 (PL 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 the 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 examined; 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 Palæechinoids. The central plate thus seems to have 
