300 



COMPARATIVE ANATOMY 



CHAP. 



lateral spine-bearing shields, and a longitudinal row of dorsal shields. On the 

 apical surface of the disc larger radial shields are found at the sides of the bases of 

 the arms : thus ten in all. On the oral side of the disc there are five interradial 

 plates which are distinguished by their great size ; these are the buccal shields. One 

 of these plates is at the same time the madreporitic plate. Mouth at the centre of 

 the lower side. Anus wanting. The ambulacral tube-feet appear on each side on 

 the arms between the ventral and lateral shields. On the lower side of the disc, close 

 to the bases of the arms laterally, there are in all ten or twenty slit-like apertures 

 the bursal apertures. These lead into blind sacs projecting into the ccelom ; these 

 are the bursae, which serve for respiration and for the reception and ejection of the 

 genital products. Development direct (viviparous and with care of the brood), or 

 with metamorphosis (free-swimming pelagic larvae). 



Order 1. Ophiurae. 



Arms unbranched, movable in the horizontal plane, usually distinctly plated. 

 Buccal shields, one of them at the same time the madreporitic plate, distinctly 

 developed. 



Fain. 1. Ophioglyphidse : Opliiura, Pectinura, Ophiolepis (Fig. 244), Ophiozona, 

 Ophioglypha, Opliioctcn, Ophiomusium. Fain. 2. Amphiuridse : Ophiadis (Fig. 245), 



FIG. 245. Ophiactis poa, Lym. (after Lyman). Disc and basal portions of t.he arms ; from the 

 oral side. 1, Ventral shields ; 2, spines on the lateral shields (4) ; 3, tentacle scales ; 5, lateral 

 buccal shields ; 6, bursal apertures ; 7, buccal shields ; 8, first ventral shield of the arm ; 9, torus 

 angularis ; 10, oral papillae. 



Amphiura, Ophiocnida, Ophiocoma, Ophiacantha, Oirfiwthrix. Fam. 3. Ophio- 

 myxidae, disc and arms covered by a thick naked integument : Ophiomyxa, Hemi- 

 curyalc. 



