340 



ECHINODERMATA 



[CH. 



The habits of the animal too are very different from those of the 

 star- fish. Instead of creeping slowly along by the action of the 

 tube-feet it springs along by muscular jerks of the arms, sometimes 

 pushing with four arms and seizing hold in front with one, some- 

 times pushing with three and hauling itself along with two. The 

 name Brittle-star is derived from the readiness with which, if 

 irritated, the animal will snap off an arm. 



As might naturally be expected, the most striking differences 

 from the star-fish are seen in the arms. No ambulacral groove 

 is apparent : the arm is encased in a cuirass consisting of four 

 series of plates, an upper row, two lateral rows each bearing a row 



,9 



FIG. 156. Section through an arm of an Ophiuroid. Diagrammatic, magnified. 



1. Badial nerve-cord. 2. Radial perihaemal canal. 3. Radial water- 



vascular canal. 4. Epineural canal. 5. Ventral plate. 6. Tube- 

 foot. 7. Pedal ganglion. 8. Lateral plate. 9. Spine. 

 10. Dorsal plate. 11. Coelom. 12. Longitudinal muscle. 

 13. " Vertebra." 14. Soft tissue supporting plates. 



of spines on its edge, and an under row (Fig. 156). On close 

 inspection the short pointed tube-feet may be seen protruding from 

 minute pores at the sides of the under row of plates. A thin 

 section of the arm reveals the fact that there really is a space 

 corresponding to the ambulacra! groove of the star-fish, but that by 

 the approximation of its edges it has become closed off from the 

 outer world so that it forms a canal, the so-called epineural canal 

 (4, Fig. 156). Above this canal, at the spot one would term the 

 apex of the ambulacral groove in a star-fish, there is a ridge of nervous 

 matter covered on the lower side by cells exactly resembling the 

 skin cells covering the nerve ridges in Asterias. This is the radial 



