of the Ophiuridse and the Ecbinidae. 437 



ami is now nothing more than the most external or terminal 

 member of the future Ophiura. Just as the first tentacles are 

 formed upon the disc itself, so is it with the first spines, ten 

 of which are seen, each being traversed by a calcareous network, 

 and situated near its tentacle. The animal has the power of 

 spontaneously moving these spines, which also indicates the 

 Ophiura. As soon as the young Ophiura has become developed, 

 it is furnished with a disc which is traversed by lattice-work 

 and incloses the stomach, and a mouth which is encircled by five 

 triangular interradial plates ; outside these plates, on the abdo- 

 minal side of the disc, there are two spines placed near each 

 other, and sufficiently large to project beyond the margin of the 

 disc. The two tentacles appear before the loss of the articu- 

 lated arms. The arm-segment itself is narrow at the root, alto- 

 gether it is longish and inflated. We meet with these young 

 Ophiurce in the upper parts of the open sea, although all traces 

 of the organization of the Pluteus have disappeared. In size they 

 are about equal to the breadth of the original Pluteus, and 

 are about two-thirds of its length. The new segment of the 

 arm is formed between the disc and the primitive segment, 

 and is furnished with two spines articulated anteriorly to the 

 sides, and two tentacles, one on each side. The young Ophiura 

 with the two arm-segments is half a line in length. Subse- 

 quently a new segment is again formed between the disc and 

 the arm and furnished with spines and tentacles. I have found 

 these young Ophiura free in the sea up to that period at which 

 their arms had four segments, and the number of spines on the 

 segments had increased to two on each side of every segment. 

 The entire animal has at this period a diameter off — 1 line. The 

 terminal segments of the arms or the primitive segments have 

 not become altered either in form or size. The subsequent seg- 

 ments differ in form, being of a polygonal shape, as is peculiar to 

 ihe segments of the arms of the Ophiura. The origin of all the 

 new segments is from the ventral surface of the disc itself, be- 

 tween the interradial plates of the disc, where the segments of 

 the arms run towards the angle of the mouth. As soon as the 

 new segment has grown beyond the disc, it forms the largest 

 of the arm-segments. Thus far the genus of Ophiw^idce to which 

 it belongs cannot be determined with certainty ; probably it is an 

 Ophiolepis, several species of which occur in the German Ocean. 

 Besides the above-described Ophiura, which in a very large 

 number of specimens may be obseiTed in all the transition stages, 

 I found another Pluteus^ i. e. the larva of another species of 

 Ophiura ; this however was only seen once. In form and in the 

 skeleton it exactly resembled Pluteus paradoxus, but the arms of 

 the larva diverged considerably more, and were much longer and 



