L. Agassiz on the Eckinodermata. 35 



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order however to complete the analogy, the starfish must be 

 represented as if swelled ; then its back answers to the dorsal 

 summit of the Echini, whence the ambulacra radiate even to the 

 mouth, by passing in the starfishes through the extremities of 

 the rays. We have thus, as in the Clypeastres and the Spa- 

 tangi, two sorts of ambulacra, one at the upper and the other 

 at the lower part of the animal. So far the analogy is com- 

 plete ; but in order to be justified in saying that the lateral 

 plates of the rays are analogous to the interambulacral plates of 

 the Echini, we must not consider the upper and lower plate of 

 each side of a ray as forming a whole, though M. de Blainville 

 seems to admit this ; but we must conceive the upper lateral 

 plate of a ray as if soldered to the corresponding upper part 

 of the next ray ; and the lower lateral plates are to be viewed 

 in the same manner, by always supposing the two sides of 

 the rays which bound the channel between two neighbouring 

 rays to be united. It is in these interambulacral plates that 

 the large thorns of certain starfishes are fotind, and these 

 thorns are analogous to the spines of the large mammellae to be 

 seen in interambulacral plates of the Echini. In the starfishes 

 there are also secondary spines surrounding the principal 

 spines in greater or less regularity. 



Besides the five oviducal plates, we observe at the summit of 

 the Echini five other smaller plates, situated at the extremity of 

 the ambulacra with which they are connected, and likewise per- 

 forated at one point, but all of the same structure as the larger. 

 Mr. Gray has given them the name of inter oviducal plates. 



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 nar- 

 row, between the large mammellated plates which bear the 

 spines, the old naturalists, fancying that they bore some re- 

 semblance to the alleys or walks in a park, gave them the name 

 of ambulacra, 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 con- 



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