538 ECHINODERMATA ECHINOIDEA 



circle of secondaries. A few deep-water forms belong to this 

 family, the type genus Scdenia (Fig. 24:0) being the best known. 

 JSTone occur in the British area. Super- 

 ficially they resemble the Cidaridae, but 

 in reality they are widely separated by 

 tlie essentially modern character of the 

 peristome. 



Fam. 4. Arbaciidae. — Endocyclica 

 with a peristome on which, as in Sale- 

 niidae, there are only ten prominent 

 plates perforated by the buccal tube-feet, 

 Fig. 240.— Dried and cleaned ^nd besides these thin irregular plates ; 



sheW of Salenia varispina, ^ ^^^ i i • ^ 



showing periproct covered external gills are present, and the auricles 

 I'tI ""'^J?^"®,,^^^'^" ^ \' consist of incomplete arches springing 



(From Wyvdle Tliomson.) ^ r o o 



from the ambulacral plates. The peri- 

 proct is covered by four vah'c-like plates. The ambulacral pore- 

 plates are separate near the periproct, but near the peristome unite 

 on the " Arbacioid" pattern {v. p. 531) to form secondary plates. 

 The interambulacral plates each carry several spines. No repre- 

 sentatives of this remarkable family are known in British waters, 

 but Arbacia is found both on the east coast of North America and 

 in the Mediterranean. It is distinguished by its conical test. 

 All the upper tube-feet are devoid of a sucker ; only those on the 

 oral surface are used for locomotion. 



Uexkiill has studied the Mediterranean species, and has 

 shown that the spines converge no matter how strong the 

 stimulus may be, and so are incapable of aiding in locomotion ; 

 also that the ectoderm is devoid of ciliation, and hence the faecal 

 matter which ftxUs on the surface of the animal is not, as in 

 other genera, allowed to fall off by the divergence of the spines 

 nor swept off by the action of the cilia. In its natural habitat 

 the wash of the ripples on the shore cleanses the animal. In 

 captivity it is liable to suffocate itself. 



Fam. 5. Diadematidae. — Endocyclica with a peristome 

 similar to that of the Arbaciidae and the Saleniidae. External 

 gills present and ten buccal tube-feet. Periproct small, covered 

 with numerous small plates. The auricles form complete arches 

 arising from the ambulacral region. Aristotle's lantern is pro- 

 vided with rudimentary Stewart's organs. The ambulacral 

 pore-plates are separated at the apex, but unite orally in 



