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L. Observations on the Growth and on the bilateral Symmetry 

 of Echinodermata. By L. Agassi z, M.D. and Professor 

 of Natural History at NeuchateL* 

 r T , HE most general character which has been usually as- 

 -*■ signed to the Echinodermata, is to have all the parts of 

 their body similar to one another, and disposed like rays 

 around a common centre : it is a character in which this class 

 has been supposed to partake with the entire division of ra- 

 diated animals. Nevertheless, on a close examination of this 

 radiated structure, we find that these rays are always dissimi- 

 lar, but in different degrees in different genera; and that they 

 are not always connected with a centre of the same nature. If 

 we trace the arrangement of parts in the Spatangi, for instance, 

 we are soon led to see that the more or less elongated form 

 of their body is caused by the position of the mouth and the 

 anus, which are placed near the two extremities of the body ; 

 and that four of the ambulacral series, and also four of the 

 interambulacral series, are pairs, and symmetrically placed on 

 the two sides of a plane, which, if extended from the mouth to 

 the anus, would divide the animal into two equal parts. The 

 5th ambulacral series, and also the 5th interambulacral series, 

 are single, and not symmetrical. The ambulacral series which 

 passes above the mouth (and which is odd, or not paired,) is 

 consequently the anterior series ; whilst the posterior part of 

 the body is occupied by an interambulacral series : it is in the 

 central line between its plates that the anus is placed. We 

 have then in the Spatangi an anterior region, distinguish- 

 able by the unequal ambulacral series, and a posterior region, 

 distinguishable by the unequal interambulacral series. On 

 the two sides of the animal the series of plates are disposed in 

 symmetrical pairs, in such a manner that there are two pairs 

 of ambulacral series and two of interambulacral on the right, 

 and two on the left. The first anterior pair, which adjoins 

 the unequal ambulacral series, is a pair of interambulacral 

 series, immediately behind which is placed the first pair of 

 ambulacral series, then a second pair of interambulacral, and 

 lastly, a second pair of ambulacral series. Behind these is 

 the uneven posterior interambulacral series. 



As to the Clypeasters, the Galerites, the Nucleolites, &c, in 

 which the mouth is central and the anus marginal or sub- 

 marginal, it is nevertheless easy to understand the position of 

 the bilateral parts, because the position of the posterior inter- 

 ambulacral series being given by the position of the anus, 



* Communicated by the Author. 

 Third Series. Vol. 5. No. 29. Nov. 1834. 3 B 



