5o6 ECHINODERMATA ECHINOIDEA chap. 



the leading factor in the evolution of Echinoidea. The spines 

 have cup-shaped basal ends, which are inserted on special pro- 

 jections of the plates of the skeleton called tubercles. The 

 tubercle is much larger than the cup, and hence the spine has 

 a great range of possible motion. The spines differ from those 

 of Starfish and Brittle Stars in being connected with their 

 tubercles by means of cylindrical sheaths of muscle fibres, by the 

 contraction of which they can be moved in any direction. The 

 muscles composing the sheath consist of an outer translucent and 

 an inner white layer. The former are easily stimulated and 

 soon relax ; they cause the movements of the spines. The latter 

 require stronger stimulation, but when aroused respond with 

 a prolonged tetanus-like contraction, which causes the spines to 

 stand up stiffly in one position ; these muscles can be torn across 

 sooner than forced to relax. Uexkiill ^ has appropriately named 

 them " block musculature." These sheaths, like everything else, 

 are covered with ectoderm, which is, however, specially nervous, 

 so that we may say that the muscular ring is covered by a 

 nerve-ring from which stimuli are given off to the muscles. 



The spines are, speaking generally, of two sizes, the larger 

 being known as "primary spines" and the smaller as "secondary." 

 In many Echinoidea these two varieties are very sharply con- 

 trasted, but in Echinus esculentus there is not such a great 

 difference in length, and intermediate kinds occur. The forest of 

 spines has an undergrowth of pedicellariae. All Echinoidea 

 possess pedicellariae, which are much more highly developed 

 than those of any Asteroid. With few exceptions all the pedi- 

 cellariae of Echinoidea possess three jaws and a basal piece. 

 This latter is, however, drawn out so as to form a slender rod, 

 which articulates with a minute boss on a plate of the skeleton. 



Of these pedicellariae there are in U. esculentus four varieties, 

 viz. (1) "tridactyle" (Fig. 225, C; Fig. 226, B) : large con- 

 spicuous pedicellariae with three pointed jaws, each armed with 

 two rows of teeth on the edges. There is a flexible stalk, the 

 basal rod reaching only half way up. These are scattered over 

 the whole surface of the animal. 



(2) "Gemmiform" (Fig. 225, A, B; Fig. 226, A), so called 

 from the translucent, almost globular head. The appearance of 



' "Die Pliysiologie des Seeigelstachels, " Zcitschr. fur Biol, xxxix. 1900, pp. 73 



et seq. 



