ECHIXODERMATA. 



conditions which admit of the body being divided into two exactly 



symmetrical halves. 

 Even these planes do not 

 exactly fulfil these con- 

 dition?, since the re- 

 maining organs are not 

 strictly symmetrical in 

 regard to such a plane. 



Very frequently one 

 of the rays differs in size 

 from the others, and 

 then we have an irregu- 

 larity in the external 

 form of the Echinoderm, 

 which renders the bi- 

 lateral symmetry visible 

 even from the exterior. 

 The pentanierous body 

 of the Echinoderm may 

 become bilateral, the 

 plane of the unpaired 

 ray forming a median 

 plane, on each side of which two pairs of equal rays are repeated. 

 We can distinguish an upper sur- 

 face (apical pole) and an under 

 (oral pole), a right and left side 

 (the two paired rays and their 

 inter-radii), an anterior end (un- 

 paired radius) and a posterior 

 (impaired inter-radius). In the 

 irregular Sea-urchins, the bilate- 

 rally symmetrical form is still 

 more strongly marked. Not only 

 is the unpaired radius of abnormal 

 size and form, and not only are 

 the angles at which the principal 

 ray and the accessory rays cut 

 each other equal only in pairs, but FIG. M%.s>-\<z<uttr (Spatangida?) , from the 



, . .-. ril .". 7 ,n. TA-\ ventral side. 0, mouth; A, anus; P, 



also in the C lypeastndea (tig. 20 / ), pores of the ambulacral foet . 



the anus is removed from the dor.-al 



pole to the ventral half of the body in the unpaired inter-radius 



FIG. 207. CIt/ptai<fer rogaccux, from the dorsal side. The 

 madreporic plate is situate in the centre and is sur- 

 rounded by five genital pores and by the five-leaved 

 rosette. The unpaired radius is directed forwards. 

 At the side is the median portion of the ventral sur- 

 face. 0, mouth ; A, anus. 



