312 ECHINODERMS OF THE BRITISH ISLES 



diameter. It is a littoral form, found especially among the 

 Zostera. 



In the British seas proper this species has not been found ; 

 but as it occurs at the Channel Islands it may be justifiable to 

 include this important type among the British echinoderms (as 

 is also done by Bell and Hoyle). It is distributed from the 

 Channel Islands to Cape Verde and the Azores and the Medi- 

 terranean ; especially in the Mediterranean it is very common. 

 Bathymetrical distribution, 0-ca, 50 m. 



III. Family Strong ylocentrotid^ 



Test without depressions or pits ; tubercles smooth. Am- 

 bulacral plates with more than three pore pairs (polyporous). 

 Gill-cuts small and obsolete. Globiferous pedicellarise without 

 lateral teeth, the outer part of the valves (the blade) a closed 

 tube ; the head articulated to the stalk by means of an extensile 

 and retractile neck (Fig. 143, 6 ; 144, 3) ; no mucous glands on 

 the stalk. Spicules of globiferous pedicellariae and tube-feet 

 C-shaped, with the ends branched (Fig. 144, 5). 



Only one genus represented in the British (and European) seas, 

 the only genus known of this family. ^ 



1. Strongylocentrotus Brandt. 



Characters the same as those of the family. Each ambulacral 

 plate carries a primary tubercle. 



Only one species in the British (and European) seas, and, on 

 the whole, in the Atlantic, whereas several species are found in 

 the Northern Pacific ; here, evidently, is the home of the genus, 

 from which one species has spread into the Atlantic. 



1 H. L. Clark includes within the family Strongylocentrotidse a number 

 of genera, mainly polyporous forms, which, in tha opinion of the present 

 author, are not more nearly related with Strong ylocentrotus but belong to 

 the families Echinidse, Toxopneustidse, and Echinometridae. It is not the 

 place here to enter on a discussion of these questions, but one main thing 

 may be emphasised, viz., that the polyporous condition of the ambulacral 

 plates is no proof of relationship, as it has quite certainly arisen indepen- 

 dently within various groups. 



