EciiiNinjrc. 143 



of such an important nature, a similar organization should 

 exist in the Echinida. 



The Ecliinida, progress by means of the joint action of 

 their suckers and spines, using the former in the manner of 

 the Asteriadte, and the latter as the Ophiurida do. Many 

 Sea-Urchins, such as live on hard surfaces, moor them- 

 selves also by means of the suckers, and thus adhere very 

 firmly to the rocks. That such is the mode of progression 

 and rest among this family I assert, not only from the 

 general belief of naturalists, but also from personal ob- 

 servation. It is necessary to do so in consequence of the 

 following statement of Professor Agassiz, which I quote 

 from the translation of his Prodromus, in the Annals of 

 Natural History. " As to the membranous tubes issuing 

 from the holes of the ambulacra, it is proper to remark that 

 they do not in any way contribute to locomotion. It is 

 rather amusing to trace the history of their advancement 

 to the honours of this function. As they are placed in the 

 Echini in bands more or less narrow between the large 

 mamellated plates which bear the spines, the old natural- 

 ists, fancying that they bore some resemblance to the 

 alleys or walks in a park, gave them the name of ambu- 

 lacra, without describing with greater precision their 

 nature and destined use. More recently the idea attached 

 to the word ambulacra was extended to the organ which 

 is placed amongst them, and has been since then most 

 erroneously considered as a locomotive organ. How in 

 fact could these small tentacula, situated as they generally 

 are in that part of the body which is never brought into 

 contact with the ground when the animal moves, and 

 overhung by calcareous solid spines, — how, I ask, could 

 these flexible tubes be used as organs of motion I It is 

 an undeniable fact, and I have often observed it myself, 



