ENDOCYCLICA 539 



" Diadematoid " fashion (p. 531) to form compound plates. The 

 interambulacral plates bear numerous primaries. The aboral 

 tube-feet are pointed, having lost their suckers. 



This family is represented (according to Agassiz) at the 

 present day by seven genera, none of which are found in British 

 waters, though one (Ccntrostephanus) enters the Mediterranean. 

 C. longispinus ^ was investigated by Uexkiill and found to be 

 distinguished by its sensitiveness to light and shade, and by the 

 quickness of its movements, which were mainly carried out by 

 its long spines. The family resembles the Arbaciidae in the 

 pointed aboral tube-feet, but in the complete auriculae it resembles 

 the next family. 



Fam. 6. Echinidae. — Endocjclica with peristome and peri- 

 proct as in the preceding family. External gills and buccal 

 tube-feet present, but Stewart's organs totally absent. Ambu- 

 lacral plates combined on the " Triplechinoid " plan (p. 531) to 

 form secondary plates. Interambulacral plates with numerous 

 tubercles. All the tube-feet have suckers. 



This family contains by far the larger number of living genera. 

 It is divided into two sub-families, viz. : — 



(«) Temnopleurinae. — Echinidae in which the plates of the 

 corona dovetail into each other by means of pits and knobs 

 along the line of suture. This sub-family does not occur in 

 British waters ; almost all the species are confined to the Indian 

 and Pacific Oceans, but on the east coast of America it is repre- 

 sented by several genera, which however inhabit deep water, e.g. 

 Tr igonocidaris arhacina. 



(b) ECHININAE. — Echinidae in which the plates meet each 

 other in straight, simple sutures. 



This sub-family is represented in British waters by three 

 genera, viz. Echinus, Sphaerechinus, and Strongylocentrotus. 

 Echinus is distinguished by having its pores arranged in arcs of 

 three, owing to the fact that its pore-plates are united in threes 

 to form secondary plates, whilst in the other two genera the 

 ambulacra! plates are composed of four or more pore-plates. Six 

 species of Echinus have been recorded from British waters, viz. 

 E. esculentus, E. acuius, E. miliaris, E. norvegiciis, E. microstoma, 

 and E. elegans. The validity of the last three is very doubtful. 



' " Der Scliatteii als Reiz fiir Centrostephanus lonffispinus," Zeitschr. fiir Biol. 

 xxxiv. 1896, p. 319. 



