THE SKELETON. 135 



The tegumentary or external skeleton exhibits many interesting modifications in the 

 Ophiurida. It consists of delicate calcareous plates or scales developed on the disk and 

 arms, the various forms of which and their mode of arrangement afiFord good characters 

 for generic subdivisions, and constitute the basis of the classification of this natural 

 Order. 



In the clothing of the disk we find the character of the tegumentary skeleton, 

 exhibiting many modifications ; for example, in Opldoderma the surface is covered with 

 numerous granules, and the sides of the arms are furnished with delicate papillae or 

 fine regular spines ; the body is smooth, and the arms resemble the body of a small 

 snake (PI. XIII, fig. 1). 



In OpUolepis (Pi. XIII, fig. 2) the upper surface of the disk is covered with naked 

 scales, having some resemblance in their arrangement to those found on the bodies of 

 lizards. In Ophiura texturata, Lamk., the orbicular disk is covered above with a number 

 of small unequal-sized plates, arranged in an imbricated manner, without intermediate 

 smaller pieces. The five pairs of large heart-shaped radial plates are placed close together, 

 the mesial suture of each pair being overlapped by a central imbricated series, which 

 occupy a position above the insertion of the arms. A circle of imbricated scales fills 

 the centre of the disk, and from its circumference a column of round and imbricated 

 scales extends through the centre of the interbrachial spaces to the border, the inter- 

 mediate area being compactly filled with small elKptical-shaped scales, closely pressed 

 together, as in Ophiolepis ciliata (PI. XIII, fig. 3). 



Ophiocoma has the disk more or less uniformly granular in the different species, and 

 even the radial plates (PI. XIII, fig. 4) are covered with minute granulations. The 

 buccal fissures are entirely bordered with hard papillae, as in Opildocoma dentata 

 (PI. XIII, fig. 4), where they form only one range between the teeth and buccal papillae ; in 

 other species they present many varieties of structure. The lateral spines of the arms 

 are smooth and very much developed. 



OphiaracJina has the disk granular throughout, except on the ovarial plates, Avhich are 

 naked (PI. XIII, fig. 5). The buccal plates are divided transversely into a small external 

 piece, and into a larger internal piece. The buccal fissures are provided with dental 

 papillae, and the arms with dehcate, conical, unequal spines, which lie close upon the 

 lateral plates (fig. 5). 



Ophiomastix has the disk covered with small imbricated scales, and it likewise 

 supports isolated cylindrical spines. The buccal plates are simple, and the buccal fissures 

 provided with hard papillae, which are grouped above the dentary pile, as in Ophio- 

 mastix annulosa (PI. XIII, fig. 6). The arms are very spinous, and the lateral plates above 

 the rows of spines support a series of claviform pieces with denticulated extremities. 



Ophiomyxa has the disk pentagonal, soft, flat, and without granules, and a few 

 obscure scales are observed in the naked integument of Ophiomyxa pentagona (PI. XIII, 

 fig. 7). The buccal plates are nearly round, being a little longer than they are wide. 



