370 Dr. Agassiz's Observations on the Growth and on 



there is no difficulty in understanding the symmetrical rela- 

 tions of the other even and odd series. We can always re- 

 cognise differences in the form of the component plates, and 

 of the ambulacra of the different pairs, which show evidently 

 the appearance of bilateral parity. These, however, are less 

 apparent in the Clypeasters than in the Spatangi. Hence 

 these data become important for the study of the internal soft 

 parts and for the appreciation of their functions. 



It might seem that in passing to the Echini and Asterice 

 (simple or ramified), whose mouth is perfectly central, and 

 whose anus, when there is one, is likewise found in the middle, 

 but upper part of the body, there would be no further traces 

 of this bilateral symmetry, and yet even here it is easy to de- 

 termine the relations of all the radiated parts and of the longi- 

 tudinal anterioposterior axis. All the radii of these animals 

 resemble one another so much outwardly, that we might 

 hardly expect to find in their arrangement traces of the bila- 

 teral symmetry which is so evident in the Spatangi, &c. But 

 if we take into account the differences which exist in the 

 structure of the plates of different series, we shall be convinced 

 that here also the symmetry in pairs is maintained under the 

 appearance of a disposition completely radiated ; in fact, we 

 find in the upper part of the disk of the Echinodermata, espe- 

 cially in the Echini, the Cidarites, &c, in the region where 

 the ambulacral and interambulacral series converge, five 

 perforated plates of peculiar form, which have been called 

 oviducal plates, and are connected with the ovaries, and five 

 interoviducal plates connected with the aquiferous system. 

 The five largest of these plates (the oviducal ones) alternate 

 with the extremities of the ambulacral series; four of these 

 are, therefore, even, and one is odd. That which is odd has 

 a porous particular structure; it is the mad reporiform body 

 of the Asterice, which equally exists in the Echini, but under 

 another form : it is always in the posterior region of the body, 

 and when it becomes imperceptible, the space where a lacuna 

 is observed still points out the posterior region of the body, 

 as the mad reporiform body of the Asterice shows that the ray 

 opposite to it is the odd anterior ray, whilst the four others 

 are even, and placed on the right and left sides of the animal. 

 The same is the case with the Solasteria?, with this difference 

 only, that in them the number of the pairs is more consider- 

 able, and that sometimes there is no uneven ray. 



In order rightly to comprehend the mode of growth of the 

 Echinodermata, it is necessary to keep in view the general 

 disposition of the solid pieces that constitute their covering: 

 they are plates of greater or less size, disposed in vertical 



