. The Danish Ingolf-Expedition. Scientific expeditions; Arctic Ocean. ECIIINOIDICA. II. "3 constantly being- larger than the inner one in eacli pair. The number of plates increases considerably; whereas at 2mm length only 2—3 pairs of plates are developed between the fasciole and the ocular plate, there are 17 pairs in a specimen of 11""" length. The fasciole 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. 4mm
. The Danish Ingolf-Expedition. Scientific expeditions; Arctic Ocean. ECIIINOIDICA. II. "3 constantly being- larger than the inner one in eacli pair. The number of plates increases considerably; whereas at 2mm length only 2—3 pairs of plates are developed between the fasciole and the ocular plate, there are 17 pairs in a specimen of 11""" length. The fasciole 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. 4mm length there was no difficulty in tracing the exact number and shape of these plates; while therefore the figures 1, 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 4-5mm (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""" and &smm they are distinctly developed in both series of these ambu- lacra (PI. XIII. Figs. 10, 12). At a size of j-$m™ 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""" 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. 6mm 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 f
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