34 L. Agassiz on the Echinodermata. 



is also generally a little larger than the others : and in the ob- 

 long Echinodermata, should they have the mouth masked 

 (which is often the case in the fossil species), the posterior end 

 of the body may still be ascertained by the circumstance that 

 an odd oviducal plate does not present itself between two of 

 the ambulacra ; for in this case, that these are always the two 

 posterior ambulacra is evident from the position of the anus 

 in those in which it is visible. We may therefore say that the 

 direction of the anterior ambulacrum is always opposite to the 

 odd oviducal plate, which is always placed towards the anus. 



The analogy observable between the structure of the madre- 

 poriform body of the Asterice, and that of the odd oviducal 

 plate of the Echini, is in these animals an important point of 

 resemblance, which may help to guide us in determining the 

 position of the parts in the first of these families, and enable 

 us to discover in that also a bilateral arrangement. In fact, 

 one of the five rays of the common Asteria is opposite to the 

 madreporiform body, and must therefore be considered as the 

 anterior ray, while the four others are placed in pairs on both 

 sides of the longitudinal axis. The case is the same with the 

 Solasteria, except that the number of rays arranged in pairs 

 is greater, and that sometimes the odd rays are wanting. 



Thus, whatever may be the external form of the Echino- 

 dermata, whether they are oblong like the Spatangi, and have 

 the mouth and the anus placed towards two marginal extre- 

 mities of the body opposite to each other, or are completely 

 of the star form, circular, or even spherical, with the mouth 

 and anus placed opposite to each other as the two poles of 

 their spheres, it is still easy to observe in them a bilateral ar- 

 rangement, and to determine which is the anterior and which 

 the posterior region, as well as to see how all the parts are dis- 

 posed in pairs on the two sides of the animal. 



M. de Blainville has already observed the relations existing 

 between the arrangement of the plates in the testa of the Echi- 

 ni and in that of the starfish, but has not formed a complete 

 or precise idea of the connexion of those parts. He is perfectly 

 right in giving the name of ambulacral plates to those which 

 form the grooving of the inferior side of the rays, and that of 

 interambulacral to those which are placed on their sides. In 



