308 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 
radial in position, the rotule and radu corresponding with 
the ambulacra. 
Four sets of muscles and one set of ligaments (PI. 
III., fig. 23) control the movements of the masticatory 
apparatus. The adductor muscles of the teeth are broad 
bands arranged as five pairs (PI. III., fig. 23; Pl. V., 
fig. 37, ad.m.t.). The two muscles of a pair are 
attached below to the interambulacral apophyses of 
the perignathic girdle. Passing upwards they diverge 
slightly, and are inserted, above, along the outer edge of 
the arch-like base (Pl. I., fig. 10) of the corresponding 
alveolus. The contraction of these muscles draws the 
bases of the alveoli outwards and downwards, while their 
apices, with their teeth, move inwards towards the centre 
of the mouth, through which they tend to project. ‘The 
opening muscles of the teeth (op.m.t.) also form five pairs, 
which run inwards and slightly downwards from the inner 
faces of the auricule to the halves of the alveoli nearest 
to them. When they contract the teeth are drawn apart, 
in the direction of the auricule, and the mouth is widely 
opened. The intermediate alveolar muscles run between 
the apposed lateral faces of the alveoli. Their contraction 
draws the latter close together, and probably assists in the 
reduction of the food in the pharynx. The muscles of the 
radi (m.7.), of which there are five, form a pentagonal ring 
which connects the five radu together; and, by their 
contraction, probably depress the whole apparatus in the 
direction of the mouth. A pair of hgaments (lig.) are 
attached to the forked peripheral end of each radius, and, 
diverging as they pass downwards, are attached to the 
interambulacral apophysis of the perignathic girdle on 
either side. The peritoneal membrane which hnes the 
body cavity is continued over the lantern, and forms five 
projecting sacs, in which the soft bases of the alveolar 
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