32 L. Agassiz on the Echinodermata. 



the two extremities, and that besides four ambulacral and four 

 interambulacral series symmetrically placed in pairs on each 

 side of a plain, which, if continued from the mouth to the anus, 

 would divide the animal into two equal parts, there is an odd 

 series of each kind. The odd ambulacral series, above the 

 mouth, must certainly be the anterior series, while, in the pos- 

 terior part of the body, it is the odd interambulacral series that 

 occupies the middle of the disc. It is also a remarkable fact, 

 that in these animals the anus is always placed amongst the 

 plates of the latter odd series. In the Spatangi we have there- 

 fore an anterior region indicated by the odd ambulacral series, 

 and a posterior region indicated by the odd interambulacral 

 series. On the two sides of the animal the series of plates are 

 disposed in symmetrical pairs, so that on the left and on the 

 right there are two pairs of ambulacral and two of interambu- 

 lacral series. The first or anterior pair, which is contiguous 

 to the odd ambulacral series, is a pair of interambulacral series, 

 immediately behind which there are placed, first a pair of am- 

 bulacrals, next a second pair of interambulacrals, and finally 

 a second pair of ambulacrals, which includes the odd inter- 

 ambulacrals, posterior and middle. Notwithstanding this ra- 

 diated and at the same time symmetrical arrangement, the se- 

 ries of plates not being of equal breadth throughout their 

 height, the Spatangi have, between the mouth and the anus, 

 a disc formed by the greater or less dilatation of the posterior 

 interambulacral series, on which they creep, and which is, in 

 fact, the lower side of the animal ; while its upper side is the 

 region towards which all the series converge above the disc. 



As to the Clypeastres, the Galerites, the Neucleolites, &c, 

 which have the mouth in the centre and the anus marginal or 

 s ubmarginal, it is still easy to judge of the position of their 

 parts, because the position of the posterior interambulacral 

 series being given by that of the anus, there can be no diffi- 

 culty in determining the symmetrical relations of the other 

 series, both the odd and the even. There are even some differ- 

 ences observable in the form of the plates and the ambulacra 

 of the several pairs ; and the bilateral parity which these ani- 

 mals still retain is rendered perceptible by this circumstance. 



At first sight it may seem more difficult to discover any 



