MADRErORIFORM TUBERCLE. 175 



brought up the extremity of an arm, with its terminat- 

 ing eye, the spinous eyelid of which opened and closed 

 with something exceedingly like a wink of derision." * 



The dismembered fragments of the Luidia continue 

 active long after their dispersion. The feet move about 

 and attach themselves to any object that comes within 

 their reach, retracting and pushing out with as much 

 vigour as they did when the creature was entire. A 

 similar irritability is often seen in the dismembered 

 portions of other of the lower animals which indulge 

 in these "destructive" propensities. 



The Star-fishes possess a curious organ, whose use 

 has not hitherto been satisfactorily ascertained by ana- 

 tomists. Its position is indicated externally by a sort of 

 wart, placed on some part of the upper surface of the 

 disc and marked with radiating striae, resembling the 

 plates of a Madrepore or the gills of a Mushroom. This 

 body is commonly called the madreporiform tubercle. 

 When the animal is cut open, a curved calcareous 

 column, composed of minute hexagonal plates, united 

 together into larger, joint-like portions, and invested 

 with a skin, is seen connecting the inner surface of the 

 tubercle with the plates about the mouth. After men- 

 tioning the various offices attributed to it, none of 

 which appear satisfactory, Mr. Forbes seems inclined to 

 regard it, with Dr. Coldstream, as the analogue of the 

 stalk of the Crinoid Star-fishes ; an opinion which will 

 be acceptable to all who delight to contemplate the 

 unities of nature. We leave the uses of this curious 

 organ untold to be determined by future observation ; 

 * Forbes's i4 Star-fishes," p. 138. 



