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The rods extend inside, and lend support to, the arms. Their number 

 varies from species to species. Thus, in the regular urchins and sand dollars 

 the fully formed larva has four pairs of arms, while the larva of heart urchins 

 has six pairs. Moreover, in heart urchins the presence of an unpaired posterior 

 rod is characteristic; equipped with complex perforations, it is termed the 

 aboral rod. 



Rods passing in direct proximity to the mouth of the larva from the dorsal 

 side are termed the preoral rods. They are simple in all species of sea urchins. 

 The rods situated behind the esophagus on the ventral side are called postoral, 

 while those lying behind the preoral ones are known as the anterolateral. The 

 anterolateral rods are always simple but the postoral ones are often complex, 

 triradiate, and with numerous perforations. The structure of the postoral rods 

 is of taxonomic significance. 



The next pair of rods, the posterodorsal, is also rather frequently complex 

 in structure; these rods are situated on the dorsal side and basally adjoin the 

 base of the postoral rods. The structure of this pair of rods is likewise of 

 taxanomic significance. 



In heart urchins, in front of the preoral rods on the dorsal side, one more 

 pair of rods develops. These rods are the anterodorsal pair and, like the 

 preoral rods, they are simple in structure. Further, in some species of heart 

 urchins yet another pair of rods, the posterolateral, may form, which diverge 

 from the upper ends of the archlike rod. 



The basal and secondary basal rods in the larvae of some groups of sea 

 urchins are fused, and form in the basal part of the larva the so-called "basket 

 structure". 



In many species of sea urchins the pluteus III stage has one or several 

 pedicellariae, also of taxonomic importance. 



1 . In sea urchins the number of arms in the larvae is a diagnostic character. 

 In larvae of the order Spatangoida there are six pairs of arms in the 

 pluteus III stage, while in those of the orders Camarodonta and 

 Clypeasteroida there are four pairs. 



2. Structure of the rods forming the skeleton: In species of the Superorder 

 Camerodonta, found in the Sea of Japan, all the skeletal rods are simple. 

 In species of the orders Clypeasteroida and Spatangoida both simple and 

 complex perforated rods occur. 



3. Presence of pedicellariaes in larvae: These are characteristic of the larvae 

 of some species of the Superorder Camarodonta. 



4. Presence of ciliated epaulettes or vibratile lobes: Ciliated epaulettes are 

 found in larvae of the Superorder Camarodonta while vibratile lobes are 

 found in larvae of the order Clypeasteroida. 



5. Distribution and color of the pigment. 



