ECHINODERMATA SPINES, ETC. 



391 



time, the poison is squeezed into the spine through the lower pores in that part of 

 it which lies within the sac, and is squirted out through the few pores or eyes at 

 its tip. 



On the fascicles of the Spatangoida, whose course has already been described 

 (p. 349), there are exceedingly numerous very small, granular tubercles carrying small 

 seta-like spines, thickened at the tip ; these are sometimes articulated, sometimes 

 immovable. Such clavulae are covered by a ciliated integument, which very 

 probably contains sensory cells. 



I. The Spines of the Asteroidea. The Asteroid body is also 

 usually covered with spines and papillae. The form and arrangement 

 of these vary so greatly that they cannot here be more fully described. 

 We must refer the reader to the principal systematic works for details. 

 Their finer structure is 

 almost entirely unknown, 

 and we have hardly any 

 knowledge of the positions 

 of the sensory organs and 

 glands which almost cer- 

 tainly occur. 



The spines are often firmly 

 connected with the skeletal 

 plates of the body wall, from 

 which they rise. Spines occur 

 most constantly at the edges of 

 the ambulacral furrows, border- 

 ing them like a palisade. They 

 are not infrequently movable ; 

 the}' can be erected, and inclined 

 over the furrow for its protection 

 (Fig. 243, p. 298). 



Many Phancrozonia, and 

 specially the Astrapectinidcc, 

 are characterised by short cal- 

 careous pillars rising from the integument, which, on their terminal flat surfaces, 

 carry a usually circular group of small, thickly crowded spines, prominences, or 

 papillae. These structures are called paxillae (Fig. 309, p. 351). 



FIG. 339. Three brachial joints of Ophiopteron 

 elegans, from the middle part of the arm, lower side (after 

 Ludwig). 5s, Ventral shields ; te, tentacle ; ss, lateral 

 shields ; 1, hook ; 2, thorny spine ; 3, supporting rods of 



the fins. 



c. Spines of the Ophiuridse. In the Ophiuridce, it is principally or 

 exclusively the lateral shields which carry spines, in a manner already 

 described (p. 355). 



These spines are mostly large, slender, and pointed, and are occasionally provided 

 with thorns. In the genera Ophiomastix, Astroschema, and Ophiocreas club-shaped 

 spines occur, together with the ordinary kinds. Over the ends of these spines the 

 epithelium is thickened, and contains glandular and sensory cells. In Ophiopteron 

 elegans, numerous small spines of peculiar structure are found on the dorsal side of the 

 disc. A short stem divides into six long pointed branches, which are connected 

 by a thin, soft membrane in such a manner as to form a kind of funnel. The whole 

 .structure somewhat recalls an umbrella turned inside out. In the same species, 

 each lateral shield carries, besides a hook and a thorny spine, ten long, slender 



