SCHUCHERTIA. 209 



which brings .about a superficial resemblance to the oral surface of Hudsonaster, 

 (2) by an outgrowth of the interradial areas which carries the madreporite oral- 

 wards. The latter character we have learnt to associate with " debris feeding." 

 The former character is probably associated with the flattening of the body, for 

 which a cause is suggested by the progressive changes of the apical surface 

 detailed below. 



The apical surface of the earliest species, 8. stellata (Text-fig. 149), is quite 

 dissimilar from that of a Hudsonaster, and even at this early stage shows a general 

 resemblance in its numerous columns, sometimes twisted, to the same surface of 

 recent members of the Paxillosa (compare the text-figure with Verrill, 1914, pi. ciii, 

 fig. 2, Luidia foliolata). Progressive changes heighten this resemblance and make 

 it more exact. Schuchert noted that the apical plates in 8. stellata were only 



Text-fig. 149.— Camera lucida drawing of the apical surface of Schuchertia stellata 



(after Schuchert). x 8. 



tumid, while in a later species, S. ordinaria, they become spicular. The account 

 given below of the latest form, 8. wenlocld, shows that by the Middle 

 Silurian very definite paxilhe have developed. The recent Paxillosa, as suggested 

 by the name, are characterised by paxillae, and a study of their mode of life 

 suggests a physiological reason for their presence. The forms live, when at rest, 

 just below the surface of the sand, and clearly might be expected to find some 

 difficulty in respiration. Their breathing organs are papulae, naked protruding- 

 patches of skin, which are found between the ossicles. The spines of the paxillae 

 act like an umbrella and keep the particles of sand away from the sensitive skin 

 and loosen the sand, so as to allow the ready ingress of streams of freshly aerated 

 water. 



It is possible that the sunk valleys between the infero-marginalia of 8. 

 wenlocld (Text-fig. 124, p. 183) were occupied by specialised spines carrying cilia 



