VARIATION IN "OPHIOCOMA NIGRA" 

 (ü. F. MÜLLER). 



By D. 0. MclNTOSH, M.A., F.R.S.E. 



FoR two months duiing the autumn of 1902 I was permittcd to occupy the 

 " Aberdeen University Table" at the West dt' Scotlaud Marine Biologieal Station 

 at Millport. This privilege I owe to the Professor of Natural History at the 

 University of Abeideen, at whose Suggestion the following piece of work was 

 priniarily undertaken as an exercise in conncction with the B.Sc. examiuation 

 in Zoology. 



The niain purpose of the present coniniunication is to give the results of my 

 examination of certain exterual features of the Brittle-star Ophiocoma nigra. My 

 first observations were made on specimens dredged in a single haul from near the 

 Tan Buoy between the Cumbraes, on September 2nd, at a depth of from tive to 

 seven fathoms, from gravelly ground well known to be frequented by brittle-stars. 

 They were gathered promiscuouslj', and on being brought to the laboratory were 

 at once placed in tanks through which tiowed a constaut stream of sea water. 

 Although other things were noted, I merely give here the results of niy attempt 

 to find : 



1. The most geueral shape of the disc. 



2. The shape and size of the coloured patch (if any) on the disc. 



3. The correlation between arm-length and disc-breadth. 



4. The percentage of specimens with au abnormal number of rays. 



5. The number of madreporic plates. 



In Order that all the above poiuts might be investigated when the animals 

 were newly killed, the method I adopted was as foUows. Specimens were selected 

 at random from one of the tanks and immediately placed in fresh water. I found 

 that in this way they died in from three to three and three-cpiarter minutes, and 

 could be handled without exhibiting the brittlenes.s characteristie of them when 



5y— a 



