CHAPTER XVII. 

 Radial Series: Echinodermata. 



As seen in the majority of adult Echinoderms the repeated 

 parts are arranged with a near approach to a Radial Symmetry 

 and it is thus convenient to consider their Meristic Variations 

 in that connexion. But it must of course always be remembered 

 that in their development these repetitions are in origin really 

 a Successive Series and not a Radial Series. The segments are 

 not all identical (as, in appearance at least, they are in many 

 Ccelenterates &c), but are morphologically in Succession to each 

 other, though there may be little differentiation between them. 



In the case therefore of Variation in the number of segments, 

 resulting in the production of a body not less symmetrical than 

 the normal body, there must be in development a correlated 

 Variation among the several members like that seen in so many 

 cases of additions to the ends of Linear Series. 



This circumstance should be kept in view by those who seek 

 in cases of numerical Variation, in Echinoderms to homologize 

 separate segments of the variety with those of the type, hoping 

 to be able to say that such a radius is added, or such other 

 missing. As in other animals, this has been attempted in Echino- 

 derms, and though I know well that in the complex subject of 

 Echinoderm morphology I can form no judgment, yet it is difficult 

 to suppose that the same principles elsewhere perceived would 

 not be found to hold good for Echinoderms also. 



All that is here proposed is to give abstracts of facts as to 

 Variation in the numbers composing the Major Symmetries. 

 It will of course be remembered that though the fundamental 

 number in Echinoderms is most commonly five, other numbers 

 also occur as normals, {e.g. four in the fossil Tetracrinus, six 

 in some Ophiurids, &c. Examples will be given of total change 

 from five to four and to six, and so on. It is besides not a 

 little interesting that of the normally 4-rayed Tetracrinus both 

 o -rayed and 3-rayed varieties should be known. 



Besides the examples of total Variation there are a few cases 

 of incomplete Variation in which there is a fair suggestion that 



