THE SKELETON. 137 



in Ophiarachna septemspinosa. The lateral parts of the triangular body forming the boun- 

 dary of the oral fissures (fissurse buccales) are armed with plates, papillae, or spines of 

 various forms in the different genera ; an inspection of the figures already cited will 

 afford a better idea of the structure and disposition of these parts than any description, 

 however detailed. The border of the fissures is either naked, as in Ophioihrix and 

 Opliionyx, or provided with hard plates or papillae (papillae buccales) arranged in a single 

 row, as in all other genera (see PL XIII, figs. 1, 3, 4, 5, 6, 7). 



The masticating surface of the maxillae in all the Ophiuridce carries teeth or papillae. 

 The teeth occupy the entire breadth of the jaw, and form a perpendicular series of dental 

 processes which extend upwards from the peristome into the interior of the disk. In 

 the genera Ophioderma (PI. XIII, fig. 1), Ophiolepis (fig. 3), OpJaomyxa (fig. 7), the teeth 

 reach to the mouth-papillae ; but in other genera they are limited to the superior part of 

 the maxillae, the lower portion being furnished with papillae, as in Ophiocoma (fig. 4) 

 and Ophiarachna (fig. 5). The teeth consist of calcareous osselets in general, having 

 a smooth border ; seldom is the surface serrated as in Opinomyxa pentagona (fig. 7). 



Upon each of the interbrachial spaces, on the under surface of the disk, are found 

 either two or four genital openings (fissurae genitales) ; when there are only two, they form 

 long slits, which lie close to the arms. In Ophiura texturata the inner extremity of the 

 slit touches the buccal plate, and the outer reaches the circumference of the disk, the internal 

 border of the fissure is finely pectinated with small spines, which fringe the aperture. 

 In the genera with four openings, as Ophioderma (PI. XIII, fig. 1), they are disposed in 

 two behind each other, in the same radial line, or they lie close together, side by side, 

 as in Ophiocnemis. 



The arms, whether simple, as in OpJduridts, or ramified, as in Aster ophy dice, 

 consist of a great number of jointed osselets, which have been already described ; 

 they are clothed externally by a series of plates arranged in an imbricated manner 

 throughout the entire ray, and disposed into dorsal, ventral, and tw^o lateral rows. 

 The dorsal and ventral plates resemble each other in form and covering, and the 

 lateral plates support the spines which arm the rays of most Ophiuridce. The spines 

 are longer or shorter in different genera; they are sometimes stout, or slender, with 

 the surface covered by fine lateral processes, as in Ophiothrix (fig. 8) ; sometimes they 

 are small, short, and closely applied to the arm, as in Ophioderma (fig. 1) ; or they 

 are large, strong, and projecting crosswise, as in Ophiocoma (fig. 4), and with additional 

 claviform processes superadded, as in Ophiomastix (fig. 6). The character of the spines 

 varies in each genus, and forms a valuable aid in the determination of species. They 

 are moved in general by the contraction of the tegumentary membrane which unites 

 them to the lateral plates. 



