VIII ECHINODEEMATA— MORPHOLOGY OF SKELETON 361 



The following table embodies this view of the oral skeleton, viz. 

 that it consists of modified pieces of the first two skeletal segments of 

 the radii (arms). 



Skeletal Segment of the arm. 



2nil (Distal) Segment of the oral 

 skeleton. 



1st (Proximal) Segment of the 

 oral skeleton. 



The two peristomal 

 plates of a radius (Fig. 



The two halves of the ! The distal portions of 

 vertebral ossicle (ambu- ■ the two oral-angle plates 

 lacral plates) (Figs. 311 ' belonging to a radius 314 avii). 

 and 314 am). j (Fig. 314 amo + ac?i). 



The two lateral shields The two lateral buccal The proximal portions 

 (adambulaoral plates) j shields of a radius (Fig. [of the oral -angle plates 



(Figs. 311 ss, and 245, 4, 

 p. 300). 



The ventral shields 

 (Figs. 245, 1, p. 300, and 

 311 bs). 



245, 5, p. 300). 



Externally visible ven- 

 tral shield of each radius 

 of the oral skeleton (Fig. 

 245, 8, p. 300). 



belonging to a radius (Fig. 

 j 314 aiih:: + adj). 

 I 



Inner ventral shield of 



the oral skeleton. 



Accessory Parts of the Oral Skeleton. 



At each oral angle (at the point where two neighbouring oral-angle plates meet 

 interradially), on the side facing the buccal cavity, there lies a vertical row of small 

 skeletal pieces, which may fuse together to form the torus angularis (Fig. 386, ta, 

 p. 486). This can'ies the teeth (D) which project into the buccal cavity. The oral- 

 angle plates themselves carry, at the edges which are visible externally, i.e. from 

 the ventral side, small spine-like skeletal pieces. Of these those which project into 

 the buccal fissures are called oral papillse ; while those which rise at the tips of the 

 oral angles, and are turned to the axis of the buccal cavity, are called dental 

 papillse. Consequently, in each oral angle, the teeth above mentioned lie dorsally 

 til the dental papillfe. 



Accessory Skeletal Plates of the Disc. 



Lower side. — The pieces already described as appearing at the surface on the lower 

 side of the disc, and which belong to the oral system (oral shields) or to the oral 

 skeleton (oral-angle plates, lateral buccal shields, ventral shields), hardly ever form 

 the whole ventral carapace of the disc. On the contrary, between the roots of the 

 arms (interbrachially or interradially) these plates leave free spaces (Figs. 245, p. 300, 

 and 314 ibr) ; these are often triangular, and are sometimes covered with plates which 

 vary in size and number, and frequently imbricate, or else they consist of soft integu- 

 ment with small skeletal granules scattered through it. These interbrachial regions 

 of the disc may be armed with spines of varying length. 



On either side of the root of each arm, on the ventral surface of the disc, there 

 are one or two fissures or slits ; if two, one iiroximal and the other distal. These 

 bursal apertures (Figs. 245, 246, pp. 300, 301, and Fig. 314) lead into the bursa", 

 which will be described later. The adradial edge of each of these slits is usually 

 supported by a single skeletal piece, the genital plate, while the interbrachial edge 

 is plated with a row of scales, which is directly continued into the plating of the 

 neighbouring interbrachial region. 



Upper (apical) side of the disc. — It follows from what was said above (j). 327) 



