ECHINODERM LARVJE. 483 



in 1907 and 1908, and reappeared in rather larger num- 

 bers in September, 1911 and 1912. It was rare in July 

 and September, 1913. Graham Kerr (6) records it from 

 Loch Sween, and says it was the least numerous of the 

 five ophioplutei found therein. Should Mortensen's 

 correlation of this larva with Ophiura ciliaris be correct, 

 it shows that the plutei of two closely allied species such 

 as Ophiura ciliaris and 0. albida may be dissimilar 

 (compare figs. 39, PI. VI, and 43, PL VII). Though by 

 no means uncommon in the deeper waters around the 

 South end of the Isle of Man, 0. ciliaris occurs much less 

 frequently in dredgings than its relative 0. albida. 

 Figs. 44 and 45, PL VII, represent an ophiopluteus 

 of which only two specimens have hitherto been observed 

 in the plankton of Port Erin. The first, from which the 

 figures were drawn, was found by Gravely in a tow- 

 netting taken on July 22nd, 1909. In spite of careful 

 search through a considerable quantity of plankton taken 

 during the same month in each succeeding year, no other 

 specimen was seen until about the same date in July, 

 1913, when, during a short visit to the Port Erin Biolo- 

 gical Station, Dr. Mortensen was so fortunate as to find a 

 second specimen. After careful examination of this larva 

 in the living state, Gravely identified it with the ophio- 

 pluteus compressus of Mortensen (12), and with this I 

 agree. All the arms are long and slender, the postero- 

 lateral ones (pto.l.a.), banded at intervals with black 

 pigment and very slightly incurved at their tips, 

 especially so. The three remaining pairs of arms are of 

 sub-equal length and nearly straight. The skeletal rods 

 of the postero-lateral arms are furnished with minute 

 spines. The transverse and end rods of the skeleton are 

 reminiscent of those of ophiopluteus paradoxus; and the 

 similarity between the two tonus is further emphasised 



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