OPHIUROIDEA 215 



tentacle scales. Mouth shields rounded pentagonal. Outer mouth 

 papilla conical, slender. Colour in life reddish- or greyish -brown. 

 Diameter of disk up to 8-10 mm. ; arms ca. ten times as long as 

 diameter of disk. 



It is a more delicate species than A . Chiajei, with which it is 

 often found associated. On account of the delicacy of the naked 

 skin of the ventral side, the whole disk, with the stomach and 

 genital organs, is very easily lost, especially by the rough handling 

 in a dredge or trawl. Such injury, however, does not kiU the 

 animal, the lost parts being easily regenerated when it is left 

 undisturbed. This, no doubt, holds good with all the Amphi- 

 urids wdth a naked ventral skin, and also some others, e.g. Amphi- 

 pholis squamata. Like the latter species, A. filiformis is phos- 

 phorescent. 



The larva {Ophiopluteus mancus) (Fig. 88, 3) is characterised 

 by lacking the postero-dorsal arms ; a recurrent rod is merely 

 indicated. Postero-lateral arms long, with a conspicuous red 

 spot at the end. It is found throughout the summer, at least till 

 October. 



A. filiformis is eaten by fishes, especially by haddock,^ plaice, 

 and dab ; it is also devoured by various Asteroids and has been 

 found in the stomach of the burrowing crab Corystes. 



In British seas this species appears to be common inside the 

 100 m. Une aU round the coasts, at least from off Plymouth 

 round north to Northumberland and the Dogger Bank on the 

 east coast. It is elsewhere distributed from the Trondhjemfjord 

 to the Sound in Scandinavian seas, and to the Mediterranean. 

 It does not occur at the Atlantic coasts of N. America or at 

 Greenland. Bathymetrical distribution ca. 5-ca. 1200 m. 



5. Amphiura borealis (G. 0. Sars). (Fig. 122, a-c.) 

 (Syn. Ophiopeltis borealis G. 0. Sars.) 



Dorsal side of disk only partly covered with scales, the inter- 

 radii being naked, as is also the ventral side of the disk. Primary 

 plates not distinguishable. Radial shields contiguous in their 

 distal part. Dorsal arm plates rounded, with an obtuse angle 



1 The curious fact has been recorded by V. Franz ("TJber die Ernah- 

 rungsweise einiger Nordseefische ", Wiss. Ergebn. N.F. 9, Helgoland, 1910) 

 that plaice living in localities rich in Amphiura filiformis feed extensively 

 on the latter, especially during night time, while otherwise the plaice is in 

 the main a day-feeding species. It appears that it is the phosphorescence 

 of Amph. filiformis that attracts the plaice to the brittle -star, to whom this 

 property is thus rather harmful. 



