THE ECHINIDEA. 489 



nus), the composition of the skeleton of the Echinidea is 

 alwaj's essentially similar to that which has just been de- 

 scribed ; but the form of the body and the relative positions 

 of the anal and oral apertures may vary very much. In the 

 Echinoida (Cidaris, Echinus) the body is spheroidal, and 

 the oral and anal apertures are opposite and central, or very 

 nearly so. In the Olypeastroida ( Clypeaster, Echinocyamus) 

 the form of the body varies from a spheroidal to an exces- 

 sively flattened and even lobed shape. The mouth remains 

 central, but the anus varies in position, from the apical sur- 

 face to the margin, or even to the oral surface, as in Echino- 

 cyamus. In the remaining division of the Echinidea, the 

 Spatangoida (Spatangus, Amphidotus, Ananchytes), the 

 form is usually a somewhat depressed oval, and both the oral 

 and the anal apertures are excentric. The madreporite and 

 the genital and ocular plates, on the other hand, remain in 

 the centre of the aboral region in all the Echinidea. 



The ambulacra present important variations in the three 

 divisions of the Echinidea. In the Echinoida they are ho- 

 mogeneous, presenting the same composition from their oral to 

 close to their apical extremities, and having the pores and 

 pedicels similar throughout. Furthermore, the ambulacra are 

 widest in the middle, and taper gradually to each extremity 

 (Echinus), or are of nearly the same size from one end to the 

 other (Cidaris). 



In many Clypeastroida, on the contrary, the oral and the 

 apical portions of each ambulacrum differ very widely, or are 

 heterogeneous. The apical moiety is usually very wide in the 

 middle, and tapers to a point marginally, where it joins the 

 oral portion. Hence there is an appearance of five petals 

 diverging from the apex ; and such ambulacra are called petal' 

 oid (Fig. 142, B). In the oral portions of the ambulacra, on 

 the contrary, the pores are either scattered widely over the 

 ambulacra], and sometimes over the interambulacral, plates, 

 forming pore-arece f or they are arranged in bands which ram- 

 ify over the interambulacral as well as the ambulacral plates, 

 giving rise to what Miiller has termed pore fascicp. In the 

 Spatangoida (Fig. 143) the ambulacra commonly present 

 the same heterogeneous character, but the oral portions are 

 not arranged in fasciae; and it not unfrequentlv happens that 

 the anterior ambulacrum becomes more or less abortive, so 

 that only four petals are obvious on the apical surface, instead 

 of five. 



The growth of the shell of the Echinidea is effected in 



