COMPARATIVE ANATOMY 



CHAP. 



390 



The structure of all Echinoid spines resembles that just described, except that 

 the pits in the socket (or acetabulum) and on the tubercle are usually wanting, 

 and with them also the axial ligament. 



The small spines of the Cidaroida are protective. They surround the anal 

 aperture, the genital apertures, and the pores of the radials (or ocular plates) ; on the 

 interambulacra, they surround the bases of the prin- 

 cipal spines like a circular palisade, and on the am- 

 bulacra they are arranged in two longitudinal rows. 

 They can be raised, and inclined towards one another 

 over the part to be protected. The smaller spines 

 have no cortical layer and no nerve ring at their 

 bases. They are always covered by the ciliated 

 jf integument, which at the tip of the spine carries 

 sensory (tactile) hairs. Each small spine carries at 

 e its base, on the side turned away from the part to be 

 protected, a whitish, transparent, ampulla - shaped 

 swelling, which seems to be caused by the presence of 

 glandular cells in the epithelium. The secretion of 

 this glandular cushion may perhaps be poisonous. 



In Ccntrostcphanus longispimis, round the anus, 

 certain short spines of a lilac colour occur ; in the 

 living animal, these constantly rotate, the tips of the 

 spines describing a circle. In the epithelium of these 

 spines there are sensory prominences, and at their 

 bases the characteristic circular ganglion. The fibres 

 of the muscular envelope are transversely striated. 



In Podocidaris, immovable spines without any 

 articulation are found, principally on the apical side 

 of the shell. 



The poisonous spines of Asthenosoma urens 

 (Echinothurid). This Echinoid is much feared by 

 fishermen and divers, on account of the acute pain 

 produced by contact with its body. Spines, whose 

 ends are swollen into shiny blue heads (Fig. 338), 

 seem to form the principal part of its poison apparatus. 

 These poison spines are arranged in regular bands on 

 Kio. 33S.-Spine with poison the interambulacra, but in the larva are found scattered 

 head of Asthenosoma urens 



(after P. and F. Sarasin), diagram- OVer other P arts as welL The axis of the s P ine is 

 matic. i, The tip of the spine ; occupied by a hollow calcareous rod with an extremely 

 2, poison sac ; 8, epithelium of fine point ; this rod, throughout the greater part of 



int^Tr^;rf?i th r its ' ei f hi is p erforated ^^ s m * i i - 



the poison head, penetrating the tudmal rows > at lts mie ^P* however, there are only 



Hpine ; 6, longitudinal rows of a ^ ew small pores or eyes. The swollen head which 



pores in the shaft (7) of the spine, surrounds the tip of the spine contains a somewhat 



large poison sac, with an aperture at its tip, through 



the spine may protrude. The epithelium lining the sac passes, at the 



aperture, into the outer epithelium of the head. The poison sac and the part of 



e spine which runs through it are filled with a clear fluid with floating vesicles 



id remains of cells), yielded by the epithelium of the sac. The sac and its 



elope of connective tissue are surrounded by a powerful muscular capsule, most 



fibres are attached on the one hand to the sac, and on the other to the part 



e spine lying below it. The contraction of these muscles causes the sharp tip 



spine to protrude through the aperture of the sac. Perhaps, at the same 



