36 L. Agassiz on the Echinodermata. 



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sidered as a locomotive organ. How, in fact, could these small 

 tentacula, with all their softness of texture, situated as they 

 generally arc in that part of the body which is never brought 

 into contact with the ground when the animal moves, and 

 overhung (de'bordes) by calcareous solid spines, — how, I ask, 

 could these flexible tubes be used as organs of motion ? It is 

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

 it is with their spines the Echini move themselves, seize their 

 prey and bring it to their mouths, by turning the rays of their 

 lower edge in different directions. But the correction of an 

 error respecting the functions of the ambulacral tubes does not 

 solve the problem relating to their nature and use. This pro- 

 blem we are yet unable to solve, as we know nothing more re- 

 specting them than that they are connected with the aquife- 

 rous system. 



The position of the anus, in the true Echini, between the 

 oviducal and interoviducal plates and those additional plates 

 which suiTound the orifice at the dorsal summit, where the 

 ambulacra terminate, has suggested the notion of a relation of 

 dependence between the ambulacra and the posterior orifice of 

 the alimentary canal. But this is far from being a correct no- 

 tion ; for the ambulacra, which have been often represented as 

 extending from the anus to the mouth, invariably converge to- 

 wards the upper part of the disc, where they appear in the 

 form of a rosette more or less distinct, while the position of 

 the anus varies considerably. It is not even median and su- 

 perior except in the Echini properly so called and in the Cida- 

 rites. In all the other Echinodermata, in which it exists, it is 

 situated between the two series of plates which form the pos- 

 terior interambulacral space and diverge more or less from 

 each other at its issue. In this case, which is the more fre- 

 quent of the two, the anus has no direct communication with 

 the ambulacral spaces. 



The position of the dental apparatus, and particularly of the 

 teeth themselves, with respect to the rays of the body, is an 

 object which I would recommend to naturalists as well worthy 

 of their attention ; for at present I myself am unable to solve 

 all the difficulties connected with the comparison of the differ- 

 ent modifications of the dental system in the several genera of 



