360 COMPARATIVE ANATOMY chap. 



ambulacral ossicle of the second skeletal segment. It is the latter 

 Avhich is provided with furrows for the nerve and the water vascular 

 rings, and Avith depressions for the oral feet (two on each piece). 

 The distal portions of each pair of oral-angle plates, which together 

 border a buccal fissure, would thus correspond with the lateral halves 

 of a brachial vertebral ossicle, not fused together. 



In viewing the under (oral) side of the disc of an Ophiuroid (Fig. 

 245, p. 300) we can easily recognise the interradially placed buccal 

 shields (scuta buccalia), which are usually large, and have already 

 been mentioned as belonging to the oral system. At the sides of 

 each buccal shield, between it and the neighbouring oral-angle plates, 

 lie two skeletal plates, which are known as lateral buccal shields 

 (scutella adoratia). That these last-mentioned plates belong to the 

 same row as the adambulacral plates (lateral shields) of the arms can 

 generally easily be seen. They are the adambulacral plates of the 

 second segment taking part in the formation of the oral skeleton. 

 The third pair of adambulacral plates is thus the first pair of lateral 

 shields in the arm. 



Again viewing the oral skeleton from the dorsal or apical side (Fig. 

 314), we see that above the ten oral-angle jjlates lie ten other plates, 

 which usually to a greater or lesser extent roof over the water vascular, 

 and the nerve furrow^s. These, the peristomal plates, thus lie upon 

 the inner sides of the oral-angle plates, i.e. the sides facing the body 

 cavity. The peristomal plates belonging to two neighbouring radii 

 meet interradially, and may fuse together to form single plates. The 

 two peristomal plates belonging to one and the same radius may, in 

 the same way, touch one another (in which case the ten plates 

 together form a closed circle), or their radial ends may remain more 

 or less apart. Accessory peristomal plates sometimes occur ; in other 

 cases these are altogether Avanting. The peristomal plates are con- 

 sidered to represent the ambulacral ossicles (halves of the verte- 

 bral ossicles) of the first segment of the oral skeleton, a view 

 which does not appear to be certainly established, chiefly because the}' 

 are in no way connected Avith the tube-feet. The tAvo pairs of tube- 

 feet of each radius of the oral skeleton, as has been pointed out, belong 

 to its two oral-angle plates. 



At the distal end of each of the oral slits radially, A'ieAved from 

 Avithout, there is, in many, indeed, in most Ophiuroidea, a plate Avhich 

 also takes part in the limitation of the oral cavity (Fig. 245, p. 300). 

 This plate can at once be recognised as the most proximal plate in 

 the row of ventral shields. It is the ventral shield of the second 

 segment of the oral skeleton. The lateral shields belonging to them 

 are the lateral buccal shields. 



In a row Avith, but dorsally to, this ventral shield, Avithin the buccal 

 ftssure, there is a second plate (Avhich, hoAveA'er, may occasionally be 

 Avanting) ; this varies greatly in size and form, and is to lie regarded 

 as the ventral shield of the first segment of the oral skeleton. 



