EOACTINID^E. 



195 



backward thrust is taken, however, by the mouth-frame rather than by the odon- 

 tophor. which seems to be of little importance. In the other line, which includes 

 Astropecten and Pentaceros, the main movements are brought about by the abductor 

 and adductor muscles working in combination with a wingedjodontophor as described 

 above. I hope in the near future to work out these points in greater detail. 



XXJ.V.V. 



MR" 



A 



m. 



Am.CK. 



Text-fig. 136.— Wash drawing of a mouth-angle plate and first ambulacral of a recent Astropecten after the 

 form had been soaked in potash and the plates isolated. Lettering as in Text-fig. 137. 



The question arises whether feeding habits do not give us a clue to the divergent 

 methods of development. The earliest Asterozoa probably lived by a combination 

 of ciliary feeding and podial feeding (see p. 12). Adaptations arose later in order 

 that advantage might be taken of the variety of food on the sea bottom. 



137 



n.r 



M.P i 



u.vr. 



D, 



■muse 



Ad-.-- 



138 



.muse 



-uj.v.r. 



Am.CK. 



A-m.Ck 



Text-fig. 137. — View of inside of the ambulacral groove in the mouth-region of a recent Astropecten. Ad., 

 adambulacral ; Am.Ch., ambulacral channel; D.muse., interdental muscle; M.P., mouth-angle plate; 

 n.r., groove for nerve-ring ; w.v.r., groove for water-vascular ring. 



Text-fig. 138. — View of inside of the ambulacral groove in the region of the mouth of Palasterina primseva. 

 Lettering as in Text-fig 137. x 18. 



The Ophiuroidea probably arose in relation to a mud environment, for a mud- 

 eating habit would necessitate a large stomach to deal with the mass of material 

 required to give adequate nourishment. Professor MacBride tells me that the 

 downgrowth of the apical interradii is simply a provision to allow space for the 



