6 



ASTEROIDEA. 



form a series of fan-like structures (fig. 5 J) ; the borders of the rays are likewise armed with 



long spines, extended transversely from 

 Fig. 5. the sides. Ihese spines are connected 



by a fold of tegument, and united to- 

 gether like the ray-bones in the fin of 

 a fish. Fig. 5, from Miiller and 

 Troschel/ who dessril.ed this gemis, 

 from the North Sea, shows the regular 

 disposition of these fan-like fasciculi 

 of spines at the borders of the am- 

 bulacral valley, and the long transverse 

 spines at the margin of the rays, both classes of spines being in this genus connected 

 together by prolongations of the common tegumentary membrane. 



B. 



A portion of a ray of Pteraster militaris, M. and T. 

 A, the under ; B, the upper surface. 



Fig. 6. 



Portion of a ray of Luidia Senegalensis, M. and T. 



B, the under surface. 



A, the upper ; 



In Luidia, the body is stellate ; the rays are long, flat, and narrow, with a single row 

 of ventral marginal plates supporting long spines ; the upper surface of the ray is 



closely set with paxillse (fig. 6 A) ; 

 the ambulacral valleys are nar- 

 row (fig. 6 £), the suckers bise- 

 rial, and two sets of spines oc- 

 cupy the under side of the 

 ray. The long, recurved spines 

 on the margin of the rays, 

 with the paxillae covering their 

 upper surface, ally this genus 

 to Astropecten. The toughness of the body and arms, in some star-fishes, is not more re- 

 markable than their fragility in others ; and the difficulty attending the capturing of an 

 entire specimen of Luidia, from its voluntary destructiveness, has been so graphically 

 recorded by my lamented colleague, that I cannot do better than quote his account. " It 

 is the wonderful power which Luidia possesses, not merely of casting away its arms entire, 

 but of breaking them voluntarily into little pieces M^ith great rapidity, which approximates 

 it to the OphiurcB. This faculty renders the preservation of a perfect specimen a very 

 difficult matter. The first time I ever took one of these creatures, I succeeded in getting 

 it into the boat entire. Never having seen one before, and quite unconscious of its 

 suicidal powers, I spread it out on a rowing-bench, the better to admire its form and 

 colours. On attempting to remove it for preservation, to my horror and disappointment, I 

 found only an assemblage of rejected members. My conservative endeavours were all 

 neutralised by its destructive exertions, and it is now badly represented in my cabinet by 

 an armless disc and a discless arm. Next time I went to dredge on the same spot, 



^ Miiller and Troschel, 'System der Asteriden,' p. 128. 



