L. Agassiz on the Echinodermata. 33 



traces of this bilateral symmetry in the Echini and the Asteria, 

 whether simple or ramified, and consequently to determine the 

 antero-posterior diameter in animals whose mouth is perfectly 

 central^ and the anus, when there is one, is likewise situated 

 in the central but upper part of the body. Nevertheless, even 

 in this case, nothing is easier than to determine the relations 

 which all the radiated parts bear to the longitudinal axis of 

 the animal. It is true that all their rays so closely resemble 

 each other in their external aspect, that it might seem of little 

 or no consequence whether we observe in their generic ar- 

 rangement those traces of bilateral symmetry which are so 

 clearly visible in the Spatangi. But if we take into account 

 the differences of structure in some plates of the several series, 

 we shall be convinced that the same symmetrical parity is pre- 

 served here also, though under the appearance of a completely 

 radiated disposition. We observe in fact on the upper part of 

 the disc of the Echinodermata, especially in the Echini, the 

 Cidarites, &c, in that region where the series of plates consti- 

 tuting the testa become convergent, some plates of a peculiar 

 form, which are called oviducal and interoviducal, and commu- 

 nicate with the ovaria and the aquiferous system. These 

 plates may serve as infallible guides in determining the regions 

 of the body. The largest of them (generally to the number of 

 five) alternate with the ambulacral series, so that they make 

 two pairs and an odd one. The anterior pair is thus placed 

 on the sides of the odd anterior ambulacrum ; the second pair 

 between the anterior and the posterior pair of ambulacra ; and 

 the fifth or odd plate, which is not always to be found in them, 

 is situated between the two posterior ambulacra, that is to say, 

 towards the anus or posterior part of the body. The structure 

 of this fifth plate is of a peculiar porous kind analogous to that 

 of the madreporiform body of the Asterm, and is found in the 

 Echini also, but under a different form. In those which have 

 but four oviducals, the plate which presents this peculiarity of 

 structure is that which is wanting, and its place is then indi- 

 cated by a depression or even a lacuna. Therefore, however 

 regular the position of these plates in the Cidarites and cir- 

 cular Echini may be, we can, nevertheless, always determine 

 the posterior extremity of their body by the odd plate, which 

 Ann. Nat. Hist. Vol. 1.— No. 1. March 1838. d 



