ECHINOIDEA 



285 



plates. The large, hoofed primary spines of the oral side scarcely 

 curved. Tridentate pedicellaria^ of one sort only, but of various 

 sizes, up to 1 mm. length of head, with valves curved and much 

 involuted (Fig. 158, 1) ; also ophicephalous pedicellarise are 

 occasionall}^ found. Colour in life a deep claret or violet colour, 

 which keeps fairly well in alcohol. Reaches a size of nearly 

 200 mm. horizontal diameter. 



In this species the remarkable joarasitic Copepod Pionodes- 



FiG. 161. — Hygrosoma Petersii; oral side. Diminished. 

 (After Koehler, Monaco, xii.) 



modes phormosomcE occurs, producing large galls on the inside of 

 the test. 



In British seas this species has been found off S.W. Ireland 

 (50° 29' N., 11° 48' W.), in a depth of 1800 m. (- Flying Fox "). 

 No doubt it will be found to have a wider distribution in the 

 deep sea off the British Islands. It is elsewhere known from the 

 Bay of Biscay, off the Moroccan coasts, and the Azores ; further 

 from the east coast of N. America and the West Indies. Bathy- 

 metrical distribution. 730-2870 m. 



