the bilateral Symmetry of Echinodermata. 3 7 1 



zones, diverging from the mouth to the periphery of the body, 

 and converging from thence to the upper centre of the animal. 

 The mouth obviously points out to us the anterior part of the 

 body ; but its ordinary position makes it look as if placed in 

 the inferior part of the animal, but in no other respect changes 

 the relations of the parts of the body to one another. We 

 distinguish three principal types in the forms of these ani- 

 mals; some are tubular (the Holothurice), others spheroidal 

 (the Echinoides), and others star-shaped (the Asterides) ; but 

 they may be reduced to two types, as the tubular form may 

 be viewed in this case as an elongated spheroid : still further, 

 these two types may be reduced to one and the same plan of 

 organization, since the enlargement or multiplication of the 

 ovarial plates at the summits of such a spheroid, and of the 

 plates around the mouth, accompanied by a contraction of the 

 interambulacral plates, would produce a star, whilst, vice versa, 

 the enlargement of the extreme interambulacral plates, and 

 the contraction of the central plates of a star, would produce 

 a spheroid. This is not a mere supposition; the essential 

 difference between the Echini and the Asterice consists in this 

 different mode of increase of parts which are essentially the 

 same. As to the general disposition of the plates in Echino- 

 dermata, there are generally twenty series of them, forming 

 ten zones, of which one half are pierced with holes, whilst the 

 other half are entire. The five zones or double series of per- 

 forated plates are called the ambulacral series; the others are 

 the interambulacral series. In the Starfish (Asteria?) the large 

 plates of the sides of two adjacent radii answer to an inter- 

 ambulacral series of the Echini, whilst each radius has a com- 

 plete ambulacral series extending from the mouth to the ex- 

 tremity of the radius, and thence back to the upper centre. 

 The middle part of each ambulacral series, or the extreme 

 point of each radius, is consequently the narrowest, and its 

 two ends, or the basis of each radius, the widest. Each radius 

 is, in fact, composed of two parts, resembling two isosceles 

 triangles united by their summits and laid one over another ; 

 whereas in the Echini the centre of each series is the widest, 

 and the extremities are the narrowest, like two triangles united 

 by their bases at the equator of the sphere. 



With respect to the original relations of all the plates which 

 form these series, we should entertain a false idea if we re- 

 presented them to ourselves as growing really in that vertical 

 succession which they seem to possess. It is, indeed, at the 

 summits of the series that the new plates are formed, but they 

 succeed each other, like leaves in plants/spirally, from one in- 



3 B 2 



