INTRODUCTION. 27 



in fossil Asterozoa. Lip to the present I have not been able to ascertain the 

 function of this groove. 



A strong interradial nuiscle (" adducteur des dents " of Viguier) stretches 

 l)etween strong dorsal ridges on the proximal extremities of neighbouring 

 anterior processes. This muscle appears to have the power of rotating the mouth- 

 angle plates upwards. The mouth-angle plates can be rotated downwards by 

 means of a muscle (the " abducteur des dents" of Viguier), which corresponds in 

 position to the transverse ventral muscles already mentioned. 



Interradially a small impaired plate may be seen. This is known as the 

 odontophor. Its exact homology has offered a fruitful field for discussion. 



The water-vascular ring excavates the anterior process in the species under 

 description just behind the ridge for the interradial muscle. 



The Mouth Parts of the Recent Ophiuroidea. 



The mouth parts of a Recent Ophiuroid are much more complex in structure 

 than are those of a Recent Asteroid. It will be well to give a straightforward 

 description of the arrangement in a typical modern form before the various views 

 as to the homologies of the mouth-ossicles in the two groups are discussed. 



The essential ossicles are five pairs of so-called "jaws." These can barely be 

 seen in oral view (Text-fig. 10), for they are almost (or entirely) covered by 

 plates, the buccal shields and lateral buccal shields, the names of which carry 

 their own significance. In order to see them clearly we must macerate a specimen 

 in weak potash and remove the shields. Plate I, fig. 3, shows a portion of a 

 specimen of Ophincoma erlnaceus which has been treated in this way. The jaws now 

 appear as stout ossicles which project well into the mouth-cavity. At their apex 

 is seen a plate, the torus, which carries stout teeth. Distally the jaws articulate 

 with the first vertebrae. 



As MacBride (43, pp. 483—484, 486) explains, the " words ' jaw ' and ' tooth ' 

 are misleading." He states that there is no evidence that the jaws of a Brittle-star 

 are ever used for crushing food. The creature " feeds on the most superficial 

 layer of mud at the bottom of the sea. This deposit consists partly of microscopic 

 alg£e and partly of decaying organic matter, and is more easily disposed of than 

 the living animals on which the starfish preys. The food is shovelled into the 

 mouth by the first two or " buccal " pairs of tube-feet in each ray." ^ 



The jaws merely carry the various plates, such as teeth and mouth-papillae, which 

 act as strainers. " By means of the muscles attaching them to the first complete 

 vertebra in the arm they can be rotated downwards so as greatly to enlarge the 

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



' Prof. MacBride iuformed me recently that the sand-stars may be observed to feed on worms, 

 which they pass along to their mouth by movements of the podia. 



