L. Agassiz on the Echinodermata. 39 



must steadily keep in view the general disposition of the solid 

 pieces which form their integument. In the Echini these are 

 plates, of larger or smaller dimensions, arranged in vertical 

 zones which diverge from the mouth towards the periphery of 

 the body, and thence seem to converge towards the superior 

 centre. In the starfishes they are plates, the smallest of which 

 are placed at the top of the rays, and the largest at the centre 

 of the channel by which the rays are separated. We mark 

 however three distinct types in the form of these animals : 

 some tubular (the Holothuria), some spheroidal (the Echini), 

 and others of the starform (the Aster ice) ; but these types may 

 be reduced to two, inasmuch as the tubular form may be con- 

 sidered an elongated spheroid ; yet further, these two types 

 may be reduced to the same plane of organization, since the 

 large growth of the plates in the summits of a spheroid com- 

 bined with the contraction of the interradial planes would pro^ 

 duce a starfish ; while, vice versa, the increase of the interradial 

 planes and the reduction of the central plates in the starfish 

 would produce a spheroid. Nor is this a mere hypothesis ; we 

 shall see hereafter that the essential difference between the 

 Echini and the Asteria consists in the different modes of their 

 growing. As to the disposition of the plates, there are in the 

 Echini twenty series of them, forming ten zones, five of which 

 are perforated and five not perforated. The five zones or 

 double series of perforated plates are called the ambulacral, and 

 the others the inter ambulacra! series. In the starfishes the 

 series formed by the solid plates are less regular and vary in 

 number : however, in those starfishes that have large plates 

 at the edges of the rays, we see that these plates correspond 

 with the interambulacral series of the Echini, while each ray 

 has a complete ambulacral series which extends from the 

 mouth along the extremity of the ray to the superior centre, 

 and the middle of which, at the extremity of the ray, is con- 

 sequently narrower than the ends : in the Echini, on the con- 

 trary, each series is broadest in the middle and narrowest at 

 the extremities. If we now attentively examine an Echinus of 

 the middle size (among those of its own species), we shall find, 

 especially in the genera Cidaris and Echinus, that the plates 

 of the several series are not so strongly attached to each other 



