192 BRITISH PALAEOZOIC ASTEROZOA. 



light on the habits of the various forms. In the starfish with adambulacral mouth- 

 frames — that is, with the mouth-angle plates as prominent wedges — the apophyses 

 are well developed. Their adradial faces carry the so-called " abductor muscles" 

 which pull the teeth of the same pair apart ; their abradial faces carry the 

 adductor muscles which pull the teeth of the same pair together. Viguier points 

 out that, in reality, the function of these muscles is the same, and that their 

 simultaneous contraction, as a ring of ten muscles round the mouth, tends to close 

 that opening. The same author shows that counter-action to these muscles is 

 afforded by the odontophor. When the abductor and adductor muscles contract 

 and so pull forward the mouth-angle plates, they must also pull forward the first 

 ambulacralia, for these are all rigidly united (see above, p. 189). The forward 

 oscillatory movement is communicated to the odontophor, which has wings fitting 

 within and firmly united to the walls of the first ampullal pore. The distal end of 

 the odontophor is attached to the firm wall of the oral interbrachial region. Thus 

 the forward pull of the mouth-muscles is opposed by the strain upon the attach- 

 ments of the odontophor to the interbrachial wall, and any relaxation of the 

 mouth-musculature will cause the mouth-frame to spring back into place. The 

 position and shape of the odontophor are shown in PI. I, fig. 1, and in Text-fig. 133. 

 It follows from this that we should expect a relationship between the strength of the 

 mouth-musculature (conveniently judged from the size of the apophysis) and the 

 odontophor. This is also dealt with by Viguier (op. cit., pp. 76 — 80), who shows 

 that in the recent Asterias and its allies, where the apophyses are barely noticeable, 

 the odontophor is without its wings and has become of less functional importance. 

 Now to apply these observations to habit. Unfortunately not nearly so much 

 is known of the habits of recent Asteroidea as one woidd wish, but we must 

 presume that the prominent wedges of the mouth-angles of the recent Asteroidea 

 have function. MacBride (43, p. 469) points out that the recent Astropectens 

 swallow their prey whole, and " the unfortunate victims, once inside the stomach, 

 are compelled by suffocation to open sooner or later, when they are digested." The 

 opening and closing of the wedges is therefore an opening and closing of a trap- 

 door. Asterias, on the other hand, pulls open the bivalves (MacBride, op. cit., 

 p. 440) and then everts its stomach. The food is digested while it is still outside 

 the mouth. The reduced wedges and permanently open mouth obviously allow 

 ready extrusion of the stomach. 



The apophyses are barely noticeable in the Ophiuroid jaw. This is because 

 the mouth can be opened in another way. The development of the ambulacralia 

 into vertebrae allows the jaws " to be rotated downwards so as greatly to enlarge 

 the mouth, and again rotated upwards and inwards, when they form an excellent 

 strainer to prevent the entrance of coarse particles. To permit this extensive 

 movement the articulatory facets on the proximal surface of the first vertebra 

 have been much modified; the median knob and pit have disappeared, and the 



