OPHIUROIDEA 



201 



Fig. 112. — Ophiactis Balli. 1. Oral side. 

 2. Dorsal side, x 8. (From Danmark's 

 Fauna. ) 



of arms triangular, contiguous, with truncated inner angle. 

 Ventral plates rounded, farther out with somewhat concave 

 outer edge ; a very obtuse inner angle. One fairly broad tentacle 

 scale. Five, farther out 4, rather short, conical arm spines. Mouth 

 shields rounded triangular ; a single small mouth papilla on each 

 side of jaw. Colour in life mostly reddish on disk, often variegated 

 with white ; arms usually banded with red-brown and whitish. 

 (The colour keeps fairly 

 well in alcohol.) Reaches 

 scarcely more than 5 mm. 

 diameter of disk; length 

 of arms ca. six times the 

 diameter of disk. 



The species is exceed- 

 ing^ variable in its colour, 

 and recalls to so striking 

 a degree, both in colour 

 and general aspect, young 



specimens of Ophiopholis aculeata, that it is hardly possible, with- 

 out a microscopical examination, to distinguish these two in reality 

 widely different forms, which are often found living together in 

 the same localities. Both of them are fond of conceaUng them- 

 selves in smaU cavities in stones, shells, etc., often so small that 



they must squeeze their body and 

 twist and turn their arms in order to 

 get in. 0. Balli is also commonly 

 found between the membranaceous 

 layers of Chsetopterus tubes. When 

 it contains ripe genital products the 

 disk generally projects rather strongly 

 between the arms. 



The larva is of the typical 

 Ophiopluteus shape. It has a com- 

 pound body skeleton (Fig. 113). 

 Only the younger stages are known, 

 the shape and specific characters of 

 the fuUy -formed larva unknown. 



In British seas this species is common off the south and west 

 coasts, and on the North Sea coasts it is known as far south as 

 Northumberland and Durham, where it is stated (Hodge) to be 

 very numerous in the magnesian limestone from deeper water ; 

 recorded also from the Rockall and Lousy Banks. It occurs 

 mainly in depths of ca. 60-400 m., but has been found down 



Fig. 113. — Body skeleton of 

 the larva of Ophiactis Balli. 

 X 275, (From Danmark's 

 Fauna.) 



r. Recurrent rod. 



