394 COMPARATIVE ANATOMY CHAP. 



definitely arranged, may occur. It is very probable that many of the 

 different forms of pedicellariae, within certain divisions, have developed 

 independently out of spines. 



1. The pedicellarise in the Ophiuroidea. In Trichaster elegans, from about the 

 thirty-sixth tentacle pore of an arm onwards, the two tentacle papillae are replaced 

 on the adoral side of each pore by two hooks movably articulated on a stem. This 

 stem also is articulated with a ventral lateral process of the corresponding brachial 

 vertebral ossicle. The skeleton of this apparatus consists of three pieces, belonging 

 to the stem and to the two diverging hooks. The hooks do not move against one 

 another, the planes of their movement being nearly parallel. On one side a flexor, 

 and on the other an extensor muscle connects each hook with the stem. In 

 Astropliyton also, similar pedicellarise are found, and in Ophiothrix fragilis the end 

 of the arm is beset with movable hooks provided with flexor and extensor muscles. 

 Similar hooks occur, further, on the lateral shields of the arms in certain species of 



Gorgonoccphalus. 



2. The pedicellarise of the Asteroidea (Fig. 342). In some groups, e.g. the 

 Astcrinidce, Solastcridce, and Pterasteridce, the pedicellarise are altogether wanting : 

 in the Astropcctinidce they are only very rarely found. v 



In the simplest cases, groups of small spines may function as pedicellarise. The 

 spines of such a group are movably connected with the body, and may be arranged 

 either in two opposite rows of four to five spines each, these rows approximating or 

 diverging ; or else at definite points of the body three or four spines stand close 

 together, forming, when they incline towards one another, a three- or four- sided 

 pyramid. Two spines even may form a group. For instance, on the dorsal surface 

 of Asterina gibbosa, spines are found sometimes isolated, sometimes united in larger 

 or smaller groups. Among these groups there are couples connected at the base by 

 a transverse muscle, and such spines can move towards one another more energetic- 

 ally than those of the other groups (Fig. 342, A to F). 



In the above cases, we have to a great extent to do with commencing or rudi- 

 mentary pedicellarise, and we recognise, in the larger and smaller groups of spines, 

 the material out of which pedicellarise with two, three, or four forceps may be 



r developed. (Of. also what has been remarked on p. 392 on the spines of the EcMnoidea 

 as organs for the seizing and conveying of prey to the mouth, and p. 390 on the 

 smaller spines of the Cidaroida.} 



The true pedicellariae of the Asteroidea usually have two blades or valves, less 

 frequently three. Stalked and sessile pedicellarise may be distinguished. 



a Sessile pedieellariae (Fig. 342, G). The two blades rise 



rectly from the integument. Each consists of a calcareous piece 



determining its shape, which may be long or short, broad or 



narrow, pointed or blunt, flat or spoon-like. The two skeletal pieces 



are directly articulated with a skeletal plate of the integument. In 



rymnastena carinifem, for example, numerous double-bladed pedi- 



llarias rise at the edge of the ambulacral furrow. The two blades 



e connected at their bases in a manner illustrated in the figure, by a 



transverse muscle, the adductor muscle. Further, each blade at its 



ter side (i.e. at the side turned away from the axis of the pedicel- 



> also connected with the subjacent calcareous plate of the 



by an opening muscle (abductor). The bases of the 



