38 L. Agassiz on the Echinodermata. 



count ; and that the position of the animal in walking or swim- 

 ming is not sufficient to guide us with reference to this point 

 is evident from the case of the Mollusca and certain insects 

 which swim on the back. Moreover, the anus of the Echino- 

 dermata is placed on the upper side, a situation different from 

 that of the same organ in other animals. I feel therefore suf- 

 ficiently warranted in considering the mouth as the principal 

 point of departure and as determining the anterior part of the 

 body. Do we not, in fact, see it placed in the fore part of the 

 Holothuria, which by habitually moving on the same side ap- 

 proximates to the Vermes, and in the pedunculated starfishes, 

 which, though fixed to the ground, uniformly put the mouth 

 forward in balancing themselves on their stems ? If, neverthe- 

 less, it were deemed advisable to adopt a different nomencla- 

 ture, and to call that side on which the mouth is placed the 

 lower side, that would make no change in the relations already 

 pointed out : that which I call the upper would then become 

 the forepart, that which I call the hind part would become the 

 upper part, and the lower would become the hind part. 



As I know not that anything has been yet published re- 

 specting the growth of the Echinodermata, I am the more in- 

 clined to think that the few observations which I have col- 

 lected on this subject will be found interesting, at the same 

 time that they will show the Echinodermata, whatever be their 

 form, to be all subject to the same mode of development. The 

 only known fact bearing on this point is, that the Echini and 

 the starfishes consist of fewer pieces when they are young 

 than at a more advanced age. It does not appear even that 

 that there is any positive limit to their growth, though the se- 

 veral species habitually exhibit a sort of middle size which is 

 proper to themselves, and from which the extremes are not 

 very far removed. It is in the Echini more than any others, 

 and especially in the Cidarites, that we find it most easy to 

 decide on the exactness of this indication, although several 

 authors appear to have sometimes forgotten it, particularly 

 when their object has been to establish new species. But we 

 have been told by no one where and how the new plates grow, 

 and in what manner they are developed. In order clearly to 

 comprehend the mode in which the Echinodermata grow, we 



