150 



ECHINODERMS OF THE BRITISH ISLES 



Some species, especially Ophiocomina nigra, but also e.g. Ophiura 

 robusta and Amphipholis squamata, are able to climb (e.g. the walls 

 of the aquaria) by means of their tube-feet. Several Ophiuroids 

 are phosphorescent, e.g. Amphiura filiformis, Amphipholis 

 squamata, Ophiopsila annulosa and aranea, OphiacantJm bidentata. 

 The cells that secrete the luminous substance are situated at the 

 base of the arm spines ; the light, therefore, appears as a double 

 series of luminous spots along each arm. As a rule the light is 

 emitted only on irritating the animal. 



The food of the brittle-stars consists of all sorts of small 

 animals, especially molluscs and worms, which are carried to the 

 mouth by means of the arms. The Euryalids appear to feed 



exclusively on pelagic animals (Cope- 

 pods, Appendicularians, etc.), which 

 they catch by means of their arms. 

 Several forms, mainly among the 

 Amphiurids, dig themselves deep 

 into the sand or mud, with only the 

 ends of the arms above the ground. 

 On account of their exceedingly 

 great numbers the brittle-stars play 

 a rather important role in the 

 economy of the sea, partly as food 

 for fishes and other animals, partly 

 as competitors for the available 

 food, especially by devouring enor- 

 mous numbers of young mussels. 

 Some species are of primary im- 

 portance as characterising animal 

 communities, as Amphiura filiformis, Amphilepis, Ophiura 

 affinis. 



The rate of growth has been examined for Ophiura texturata ; 

 it takes two years to reach sexual maturity, but three, or probably 

 more years to reach full size. More recently Grieg has shown that 

 Ophiomusium lymani takes about three years to reach full size, 

 and he thinks the same to hold good for most other deep-sea 

 Ophiuroids of the North Atlantic. All brittle-stars have a strong 

 regenerating power. Not only are the arms easily regenerated, 

 when lost, but also parts of the disk and the inner organs. Some 

 Amphiurids, especially such as have the ventral side of the 

 disk naked, without scales, are apt to throw off, when in any way 

 hurt, the whole disk with the stomach, the burscTe, and the genital 

 organs, afterwards regenerating all of it. Most brittle-stars are 



Fig. 86. — Diagrams showing the 

 walking of a Brit tie -star : 

 in A, with the unpaired 

 arm directed forwards ; in 

 B, directed backwards. 

 The figure in outline shows 

 the place of the Brittle - 

 star after the leap. (After 

 bstergren ; from Dan- 

 ^nark's Fauna.) 



