﻿76 



Proceedings of the Royal Physical Society. 



dozen — a probable indication that this animal is not 

 gregarious in habit like some other Echinoderms, for 

 example, Ophiocoma nigra. Consequently, it took a 

 lengthened period to collect six hundred of them, and 

 even that number could not have been obtained but for 

 the kindness of Mr Alex. Gray and Mr Thomas Hill, who 

 were then in charge of the Station, and who kindly pre- 

 served many for me. As to the relative frequency of certain 

 Echinoderms in the Clyde, I have no actual figures, but I 

 may state my impression that at least ten specimens of 

 Asterias riibens, and more than double that number of 

 Echinus esculentus, are dredged for each Solaster imfioosus. 

 On the other hand, about ten specimens ol' Solaster papposus 

 are found for every Asterias glacialis, and more than double 

 that number for every Luidia fragilissinia. 



Unlike most Echinoderms, which show in adult form a 

 perfect pentamerous symmetry, Solaster papposus is well 

 known to have a variable number of rays, and the following 

 extract from Forbes's British Starfishes may be taken as 

 giving the commonly accepted view: "The rays are generally 

 twelve or thirteen in number, sometimes as manv as fifteen." 

 The following Table, which represents the facts as found by 

 me, is self-explanatory. 



Variation Table. 



No. of Arms, 



9 



10 



11 



12 



13 



14 



15 



16 



Total. 



No. of Specimens, . 



1 



8 



19 



154 



351 



72 



6 



1 



612 



Percentage, . 



0-16 



1-3 



3-1 



25-] 



57-4 11-8 



0-98 



0-16 



100 



It will be observed that the total range of variation as 

 here shown is froui nine to sixteen rays. The solitary nine- 

 rayed specimen was dredged from shallow water with muddy 

 bottom opposite Mount Stuart, on the east side of Bute. 

 I have preserved this specimen, which is still quite un- 

 damaged. It has none of the characteristics of the Purple 

 Sun- Star (Solaster endeca), but is distinctly a nine-rayed 



