OPHIUROIDEA 



207 



of the arms protruding above the ground- surface for catching 

 the food ; this consists in the main of detritus, but also all sorts 

 of small organisms, Avorms, young molluscs, and echinoderms, etc., 

 are captured and are transferred to the mouth by means of the 

 tube-feet. The^^ seem to exercise but rough selection in picking 

 out detritus or living forms of food. They may occur in very 

 great numbers, the bottom being completely interwoven by their 

 arms, as with a web, and they doubtless play a very important 

 role in the economy of the sea ; some of the animal communities 

 of the sea are especially characterised by Amphiurids {e.g. in the 

 Moray Firth). As food for fishes they are of small importance. 



Most of the species live on mud bottom ; one of the European 

 species, Acrocnida brachiata, lives in sand, burying there as do 

 the other species in the mud. Another, Amphipholis squamata, 

 is very common under stones at 

 low water. In accordance with 

 these habits the Amphiurids do 

 not as a rule move freely about, 

 as do most other Ophiuroids, and 

 when placed on a hard sub- 

 stratum do not push themselves 

 forward, like other Ophiuroids, 

 but move in this way that one 

 arm is extended and then snake- 

 like contracted, the outer end 



fastening to the substratum ; the body is thus dragged for- 

 wards, the four other arms being drawn after. When extrud- 

 ing the genital products they assume a very curious position 

 (Fig. 118), raising themselves some centimetres above the bottom, 

 the disk resting on the proximal part of the arms as on five 

 columns; the distal part of the. arms remains in the ground. 

 When the genital products are emptied out the animal again 

 sinks down in the ground. 



Several Amphiurids are viviparous and hermaphrodite, 

 among them two British species, Amphiura boreaJis and 

 Amphipholis squamata. Others have a typical Ophiopluteus 

 larva ; the development is, however, knoA\Ti only for one species, 

 Amphiura filiformis. Two species are knowTi to be phosphorescent, 

 viz. Amphiura filiformis and Amphipholis squamata. 



In their general appearance the Amphiurids are very much 

 alike, and, especially within the genus Amphiura proper, it is 

 hardly possible to distinguish the species without microscopical 

 examination. It is therefore important to collect abundant 



Fig. 118. — An Amphiiira in its 

 spawning position. (After Th. 

 Mortensen, Scand. Ech.) 



