2G3 



ECIUNODERMATA. 



conditions which admit of the body being divided into two exactly 



symmetrical halves. 

 Even these planes do not 

 exactly fulfil these con- 

 ditions, 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 pentamerous 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 



(unpaired inter-radius). In 



irregular Sea-urchins, the bilate- 



rally symmetrical form is 



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 



Fro. 207. CJypsaster rosaceut, from the dorsal side. The 

 madreporic plate is situate in the centre and is sur- 

 roundad 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. O, mouth ; A, anus. 



each other equal only in pairs, but FIG. ws.Srhnaster (Spafan/jiiia), frorr. the 



., ,-,, , . 7 / r- \f\-\ ventral side. O, mouth; A, anus; P, 



also in the Clypeastmdea (fig. 20 1 ), pores of the ambulacral fcet . 



the anus is removed from the doival 



pole to the ventral half of the body in the unpaired inter-radiu<=, 



