358 COMPARATIVE ANATOMY CHAP. 



plate receives the terminal tentacle in a channel on its ventral side, it 

 is important to note that " the terminal plate in the Ophiuroidea also 

 originally forms a channel opening downwards, and only later closes 

 to form a ring." 



The relation between the terminals and the developing brachial 

 skeleton is the same in the Ophiuroidea as in the Asteroidea. The 

 oldest skeletal segment is the one lying most proximally (orally), and 

 of the following segments the more distal are always the younger. 

 The plates which compose each newly appearing skeletal segment 

 always arise at the end of the arm, on the proximal side of the ter- 

 minal, which thus remains at the extreme tip of the arm. 



When we consider the paired elements of the vertebral ossicles and the relative 

 positions of the skeletal plates and the water vascular system, we are able to estab- 

 lish the following homologies between the components of the brachial skeletons of 

 the Ophiuroidea and Asteroidea. 



OPHIUROIDEA. ASTEROIDEA. 



The two lateral halves of the vertebral Ambulacral ossicles. 



ossicles. 



Lateral shields. Adambulacral ossicles. 



Ventral shields. Not represented. 



(b) The Oral Skeleton. 



The most important and constant plates of the oral skeleton, in 

 the Ophiuroidea, as in the Asteroidea, are the specially modified 

 proximal plates of the brachial skeleton. The most satisfactory view 

 which has been propounded as to the morphological worth of the 

 oral skeleton is that it consists essentially of the ambulacral ossicles 

 (the halves of the vertebral ossicles), adambulacral ossicles (lateral 

 shields), and ventral shields of the first and second proximal skeletal 

 segments of the arms. 



If we look at the oral region of any Ophiuroid from without, i.e. 

 from the free oral surface of the disc, or from within, i.e. after removal 

 of the apical cover of the disc and the viscera, we see the mouth in the 

 centre of the disc as a rosette-like, or star-shaped, aperture. The slits 

 arranged radially round the centre are called the bueeal fissures. Be- 

 tween them lie the triangular oral-angles (Figs. 245, p. 300, and 314). 

 Five pairs of large plates form the frame surrounding the mouth ; 

 these are the oral -angle plates (Fig. 314). At the interradial 

 angle of each of these, i.e. the angle which projects towards the 

 centre of the oral aperture, two neighbouring angle plates meet. 

 Each angle plate has, on the side facing a buccal fissure of the oral 

 aperture, two depressions for receiving the first tube-feet which have 

 shifted into the oral aperture, and are known as oral tube-feet, or oral 

 tentacles. There are often in addition, in the dorsal side (that facing 

 the body cavity) of the circle of oral-angle plates, two circular furrows 



