ECHLNOIDEA. 



213 



Exocyclica the shell is flattened and the ambitus is oval, or 

 roughly pentagonal (Fig. 152) or heart-shaped (Fig. 153). In 

 these forms the body is lengthened in the antero-posterior axis,* 

 and a distinct bilateral symmetry is apparent ; but as in other 

 similar cases amongst Echinoderms this symmetry is delusive, 

 for, when it is closely examined, it is found to be almost entirely 

 confined to the shape of the animal and not to affect the arrange- 

 ment of the organs. 



In these bilateral forms, the mouth may remain central or 

 nearly so (Clypeastroida, Fig. 152), or it may shift towards the 

 anterior edge of the shell 

 in the direction of the 

 anterior radius (Spatan- 

 goida, Fig. 153 ; in the 

 Pourtalesiidae it actu- 

 ally is at the front end 

 on the ambitus). The 

 apical system also, 

 though it remains more 

 or less in the centre of 

 the upper surface, may 

 shift in front of or be- 

 hind the central point. 

 It thus comes about, as 

 a glance at the figures 

 will show, that the radii 

 and interradii are no 

 longer similar : they 

 differ in length accord- 

 ing to the position of 

 the mouth and apical system, as well as in other particulars 

 to be shortly noticed. 



In Palaeechinoids, Cidaroids, Diadematoids, most Holec- 

 typoids and some Spatangoids, the ambulacra are similar 

 throughout their whole course. In Clypeastroids and in most 

 Spatangoids, however, the ambulacral plates present a peculiar 

 modification, described as petaloid, on the upper surface. This 



* The antero-posterior axis is the axis passing through the radius 

 (No. ///) to the left of the madreporite which is called the anterior radius, 

 and through the interradius opposite to this, the posterior interradius 

 (No. V. I). 



FIG. 152. Clypeaster rosaceus from the aboral side 

 (from Claus). The madreporic plate is in the centre 

 and is surrounded by five genital pores and by the 

 5-leaved rosette. The anterior radius is directed up- 

 wards, and the posterior interradius downward. At 

 the side is the median portion of the oral surface 

 showing the central mouth 0, and the anus A in the 

 posterior interradius. 



