XVI 



PAXILLOSA 



469 



unfortunate victims, once inside the stomach, are compelled by 

 suffocation to open sooner or later, when they are digested. 1 



Many interesting experiments have been made on Astropecten 

 by Preyer and other 

 investigators, but one im- 

 portant fact 2 has escaped 

 their notice, that Astro- 

 pecten, when at rest, lies 

 buried in the sand, whilst 

 the centre of the aboral 

 surface is raised into a cone 

 which projects above the 

 surface. On the sides of 

 this cone the few papulae 

 which this species possesses 

 are distributed. This rais- 

 ing of the aboral surface 

 is obviously an expedient 

 to facilitate respiration. 

 It loosens the sand over 

 the region of the papulae, 

 and thus allows the water 

 to have access to them. 

 We can thus understand 

 how the restriction of the 

 papulae to the dorsal sur- 

 face, so characteristic of the 

 Paxillosa, is not always as 

 Sladen imagined, a primi- 

 tive characteristic, but often 

 an adaptation to the bur- 

 rowing habits which in all 

 probability are character- 

 istic of the whole order. 

 In both Luidia and Astro- 

 pecten Cuenot has described short spines covered with cilia in 



.adatnb. 



Fig. 



202. Oral view of Psilaster acuminatus. 

 x f. adamb, Adambulaeral spines ; pax, 

 paxillae ; pod, pointed tube -feet devoid of 

 sucker. (After Sladen. ) 



1 Schiemenz (reference on p. 440 n.). 



2 This fact was discovered by Dr. E. J. Allen, Director of the Plymouth Bio- 

 logical Station, who pointed it out to the author during the batter's sojourn at the 

 station in 1899. 



