196 PHYLUM ECHINODERMATA. 



the adjacent segments, except in those forms in which the 

 skeletal plates are deficient (Cladophiurae, some Strep tophiuridae) 

 and the arms have a soft integument which contains only 

 small skeletal pieces. The lateral arm-plates, which are generally 

 compared to the adambulacral plates of Asteroids, carry spines, 

 the others do not. The tube-feet emerge through openings 

 between the under and lateral plates, one pair in each arm- 

 segment. At the edges of these apertures are small scales. 



The ambulacra! or vertebral ossicles arise as separate pieces, which 

 generally fuse together in pairs. In certain deep sea forms (Ophiohelus) 

 each of these has the form of a curved rod joined to its fellow at each end. 

 The successive ossicles movably articulate with one another and are 

 attached by muscles (Fig. 140, 13 and 16). The articular surfaces vary 

 in form and may develop processes and pits analogous to the zygosphenes 

 and zygantra of an ophidian vertebra ; but sometimes the articulating 

 surfaces are simple and the arms have a greater power of movement 

 (Astrophyton, etc.). The radial water-vascular trunk lies in a groove on 

 the lower side of the ambulacral ossicles, and its branches in passing to 

 the tube-feet have a curved course through these structures (Fig. 140). 



Spines are present on the lateral plates of the arms, and 

 occasionally on the upper surface of the disc and on the lower 

 surface between the arms. In Ophiopteron elegans some of the 

 arm-spines of each arm-joint are united together by a thin 

 transparent membrane, thus forming a series of lateral fins. 

 Pedicellariae of the ordinary type are not present, but in some 

 forms (Cladophiurae) movable hooks, generally articulated to a 

 pedicle, are found on certain parts of the arm. The hooks which 

 occur in pairs are not opposed but move parallel to one another. 

 Such modified pedicellariae are found in Astrophyton, Ophio- 

 thrix fragilis, Trichaster elegans. The absence of true pedi- 

 cellariae in Ophiurids would appear to militate against the view- 

 that pedicellariae play an indispensable part in keeping the 

 skin of Echinoderms clear of foreign growths and debris. 



The lower skeletal pieces of the disc constitute the oral skeleton 

 and are very complicated. They comprise the proximal ambu- 

 lacral and adambulacral (lateral) brachial plates, and the inter- 

 radial buccal shields, on one or all of which are the water pores 

 (see below), and a number of accessory pieces which belong to 

 the ambital skeleton. 



The integument on the lower side of the disc between the insertions of 

 the arms is either soft and contains small isolated skeletal pieces or 

 granules, or is provided with a layer of imbricating plates. The bursal 



