INTRODUCTIOi\. 15 



oral face is also slightly turned to the observer so that the articulatory face for the 

 dorsal longitudinal muscle has been rotated into view. The ambulacral canal is 

 seen on the extreme right. This is followed by the depression for the ventral 

 cross-muscle. On the extreme left is a depression for the main vertical muscle 

 between the ambulacralia and adambulacralia {ri<h' siipni, p. 10). I'roxinial to 

 this is a second smaller depression for the attachment of the aml)ulacral to a 

 contiguous adambulacral. 



The manner of the attachment of the ambulacralia to the adambulacralia can 

 be better seen by reference to Text-fig. 7 Avhicli represents two succeeding 

 adambulacralia in apical view. The base of the ambulacral fits on to two con- 

 tiguous adaml:)uhicralia. Text-fig. 9 shows the same fitting in side view. The 

 main attachment (that under the apophysis), however, is the one wliicli connects 

 the ambulacral to the adambulacral with which it corresponds in the series. It is 

 necessary to emphasise this fact, as the descriptions frequently given imply that in 

 the Recent forms the ambulacral has e(i[ual attachments to two neighbouring adam- 

 bulacralia. Schondorf has shown that in the older Palfeozoic Asteroidea each 

 ambulacral is associated only with its own adambulacral. The small extra 

 articulation, evolved later, helps the general strengthening of the ambulacral arch 

 by providing a firmer base of attachment. 



The adambulacralia after maceration readily separate. The hollow articulations 

 (Text-fig. 7) which serve for the insertion of the longitudinal muscles, can 

 readily be seen. Similar articulations can be observed in many fossil forms. 



The Structure of the Arm in Recent Ophiuroidea. 



(i) Tha General Structure. — At first sight there is little in common between the 

 arm of an Asteroid and that of an Ophiuroid. If we examine Text-figs. 10 and 

 12 it is seen that the arm is sharply marked off from the disc and is covered with 

 a complete sheathing of plates. These plates are four in numljer and termed the 

 dorsal, side, and ventral shields. There is in consequence no open ambulacral 

 groove. The tube-feet are small, and project through the above sheathing at 

 points near the junction of the side and ventral shields. 



If the shields be stripped away a number of small ossicles (Text-figs. 14 — 17) 

 may be seen, which on account of their shape and the character of their articulatory 

 surfaces are known as vertebrae. The vertebrse allow that great flexibility of the 

 arm which is characteristic of the Ophiuroid (snake-tailed) class. Their highly 

 modified structure is specially adapted to the animal's mode of life. As MacBride 

 has pointed out (43, p. 477), the Ophiuroids progress, not by movements of the 

 tube-feet as do the Asteroids, but by writhing movements of their arms. The 

 large spines attached to the side-shields (Text-fig. 11) enable them to get a grip 



