194 BRITISH PALEOZOIC ASTEROZOA. 



muscles of Ludwig (I). musc 2 of Text-fig. 134, p. 193). Contraction of these 

 muscles elongates the triangle, and as movement backwards is prevented by the 

 stiff mouth-frame, the jaws must move forward. There is a muscle at the apex 

 of each triangle. This is the interradial muscle of Ludwig, and obviously 

 serves to bind the jaws so that they do not split apart when the " thrusting " 

 muscle is at work. In searching for the origin of these muscles I have been 

 led to the discovery of a similar apparatus in, at any rate, some Asteroidea. 

 It has been supposed that in the Asteroidea there is but one binding muscle 

 between the basal processes of the mouth-angle plates of the same interradius. 

 Text-fig. 135 shows that in Solaster there are two such muscles separated by 

 a ridge. The upper muscle of the figure corresponds to the internal interradial 

 muscle of the Ophiuroidea and is a " binding " muscle. The lower muscle is the 

 external interradial muscle of the Ophiuroidea and obviously has a " thrusting " 

 function. In Solaster, however, the muscles are almost at right-angles to the 

 position in the Ophiuroidea — and the " jaw " triangle is not thrust so much 

 forwards as upwards away from the mouth-cavity. 



Certain of the Palaeozoic Asterozoa show a considerable enlargement of the 

 internal muscle, suggesting that in some forms this muscle could contract and 

 move the mouth-parts by its own efforts. The wide gape between the mouth- 

 angle plates of Schuchertia and Pal'asterlna (see below) must have been occupied 

 by such a muscle. In these forms the position of the external and internal 

 muscles is intermediate between that occupied in Solaster and that iu recent 

 Ophiuroidea. The internal muscles would obviously act in the contrary way to 

 the external muscles — that is, they would separate the bases of contiguous 

 mouth-angle plates, shorten the " mouth-angle plate " triangle, and so tend to 

 make the mouth-aperture wider. 



If we try, therefore, to follow the evolution of the movements of the 

 mouth -parts, we obtain a result somewhat as follows : The earliest mouth-frame 

 is somewhat loosely built, e. g. that of Platanaster. The chief movements are 

 brought about by the external and internal interradial muscles, which thrust the 

 angle-plates backwards or forwards. The movements are limited by the odon- 

 tophor. Later, both in the Asteroidea and the Ophiuroidea, the mouth-frame 

 became stiffened to help to sustain the backward thrust. The stiffening was 

 brought about by the enlargement of the apical portion of the first ambulacral, or 

 by the fusion of the proximal portions of the first few ambulacralia. The adductor, 

 muscles between the apophyses of the mouth-angle plates assisted the mouth-closure, 

 but it is probable that abductor muscles were absent. The earliest Ophiuroidea 

 opened and closed their mouth in this manner ; when later their ambulacralia 

 became modified into vertebras, freer movements were allowed by the rotation of 

 the jaws on the vertebral faces. The Asteroidea probably developed along two 

 lines. In one of these, that including Brisinga, the original movements persist ; the 



