VIII 



ECHINODERMA TABOD Y MUSCULA TURE 



473 



4. An upper transverse muscle connects the two opposite ambulacral plates of 

 one and the same skeletal segment, on their apical side (that turned to the body 

 cavity). These muscles, by their contraction, widen the ambnlacral furrow (Fig. 

 382, 3 and 6). 



5. A lower transverse muscle connects the two ambulacral plates of a segment 

 on the lower side, which is turned to the furrow. These muscles, by their contrac- 

 tion, narrow the furrow. 



The musculature of the oral skeleton (Fig. 381) is arranged as follows : 



FIG. 381. Oral skeleton and basal part of the bracMal skeleton of Pentaceros turritus, 

 with the musculature of these parts (after Viguier). From within. 1, First adambulacral plates ; 

 I-IV, first to fourth ambulacral plates ; or, orals ; 2, dorsal longitudinal muscles ; 3, dorsal trans- 

 verse muscles (for opening the ambulacral furrow) ; 4, interbrachial pillars ; 5, muscle apophyses 

 of the first adambulacral plates ; 6, facets of the ambulacral plates for the attachment of the dorsal 

 transverse muscles ; 7, ditto for the attachment of the dorsal longitudinal muscles ; 8, transverse 

 muscles between the first adambulacral plates (teeth); 9, dorsal transverse muscles between the first 

 pair of ambulacral plates ; 10, dorsoventral muscle ; 11, stone canal ; 12, crossed ligament ; 13, ab- 

 ductor dentium ; 14, adductor dentium ; a. aperture for the first ambulacral feet. 



1. A single or double radial muscle connects the distal ends of the first adam- 

 bulacral plates (teeth, 1 in Fig. 310, p. 352) of one and the same radius (Fig. 381, 13), 

 and opens these plates. 



2. A muscle, which connects the distal ends of the first two adambulacral plates 

 of two neighbouring radii, and is therefore interradial. These interradial muscles, 

 by contracting, close the pairs of teeth (Fig. 381, 14). This closing action 

 is assisted by a transverse muscle, which joins the opposing edges of each pair of 

 teeth (8). 



3. The first pair of ambulacral plates of a radius, like all succeeding pairs, are 

 connected together by means of a dorsal transverse muscle (Fig. 381, 9). 



