264 THE STELLER01DEA 



fissure. The smaller piece is the jaw, which unites with the jaw 

 of the next arm to form the angle of the oral segment. Each jaw 

 has a depression, in which rest the oral tube-feet, and is notched by 

 a groove for the circumoral nerve ring. 



The union of two adjoining jaws is strengthened by a small 

 plate at the apex, known as the " jaw plate " or torus angularis. 

 This plate supports the teeth, of which, in Ophiura ciliaris, there are 

 five in each segment. A series of similar processes occur along 

 the side of the jaw, projecting into the buccal fissure ; these are 

 the oral papillae (mouth papillae or buccal papillae), of which, in 

 Ophiura ciliaris, there are ten or more in each series. 



The angle between the two jaws of one segment is occupied by 

 a shield-shaped plate, known as the " buccal shield " (oral plate, 

 mouth-shield, or scutum buccale). These plates are interradial in 

 position ; the smaller plates corresponding to them, but radial in 

 position, are the first ventral arm-plates. Between the mouth- 

 frames and the buccal shields are five pairs of long, bar-shaped 

 " peristomial plates," which cross the interradial spaces from arm 

 to arm ; they cannot be seen from without, as they are covered 

 by the shields, and by two plates beside the latter, known as the 

 lateral buccal shields (or scAita adoralia). 



The oral skeleton of the Ophiuroid is therefore very different 

 from that of the Asteroid. But both are formed by the modification 

 of similar parts, viz. the ambulacral and adambulacral ossicles of 

 the arm segments. In the Ophiuroid the supposed homologies of 

 the principal parts are as follows : 



Vertebral ( = ambulacra 1 Lateral arm-plates 



ossicles). ( = adambulacral ossicles). 



1st arm segment peristomial plates jaws 



2nd mouth-frames lateral buccal shields. 



The remaining elements in the skeleton of Ophiura belong to 

 the exoskeleton. On the upper or aboral side of the disc there are 

 five pairs of large and somewhat pyriform plates known as " radial 

 shields " (Fig. XXIII. r.s). Between them are a few smaller plates, 

 and the rest of the disc of 0. ciliaris is covered with small, 

 irregular, imbricating scales. 



On the lower surface of the disc there are two pairs of long, 

 thin plates beside the bases of the arms. A narrow cleft, the 

 " bursal slit " (known also as the bursal aperture, genital slit, 

 genital cleft, and ovarian aperture), separates these plates. This 

 slit leads into the bursal cavity, into which the generative products 

 are discharged from the gonads. Of the two bar -like plates 

 bounding the bursal slit, the larger is the "genital plate," and the 

 smaller is the " genital scale." 



The Alimentary Canal of the Ophiuroids is simpler than in 



