OPHIUROIDEA 



205 



form a conspicuous 10-radiate star. Radial shields as a rule 

 completely concealed. On the ventral side the granules or 

 spinelets may be wholly wanting. Dorsal plates of arms trans- 

 verse-oval, wholly separated by the surrounding small plates. 

 Ventral plates almost rectangular, usually separated by a small 

 depression. Six to seven short, thick, more or less compressed, 

 outstanding arm spines. The lower one the shortest ; in outer part 

 of arm transformed into a hook with usually one small thorn 

 below the large terminal claw. One rather large tentacle scale, 

 exceptionally two at the proximal pores. Three, rarely two, mouth 

 papillae ; the infradental papilla usually very small. Mouth 

 shields oval, but rather variable in shape. Colour (which keeps 





1 <e-CTY^ 



Fig. 116. — Ophiopholis aculeata. (From Danmark's Fauna.) 



1. Part of oral side. 2. Of dorsal side. 3. Two arm joints from distal part of arm, in 

 side view, showing the lower arm spine transformed into a hook. 1 and 2. x 4. 3. x 8. 



well in alcohol) mostly red, often variegated ; more rarely the disk 

 may be bluish. Arms often dark banded. Grows to a consider- 

 able size, ca. 15-20 mm. diameter of disk. Arms ca. four times 

 that length. 



It is a robust form, with relatively short, coarse arms. Its 

 movements are very slow. Like Ophiothrix, Ophiopsila, and 

 Ophiactis Balli, it is fond of nestling in hollows and crevices of 

 stones, shells, etc., being able to squeeze its disk and twist its 

 arms so as to conform with the irregularities of the crevices. Its 

 food appears to be mainly detritus. It is eaten especially by the 

 cod. 



The eggs are very small and numerous ; when full of mature 

 sexual products the interradial spaces of the disk are rather 

 swollen and bulging out between the arms. Its larva is a typical 



