No. 560] 



CAMBRIAN HOLOTIIUIUA XS 



493 



pentaradiate symmetry is often brought forward as a 

 character of the highest importance; but it is the result 

 not of a class peculiarity, but of simple mechanics: the 

 somatic divisions in the echinoderms are marked by lines 

 of weakness; hence the divisions of the body must be 

 uneven in number, so that no line of weakness will go 

 straight through the body, thus subjecting the animal to 

 danger through a shearing strain ; when the somatic divi- 

 sions are by lines of extra strength, as in the ccelenterates, 

 the divisions are always equal, as in this case the con- 

 tinuation of a line of strength directly across the body 

 gives added rigidity. 



We know enough about organic life at the present day 

 to be somewhat sceptical when new phyla are proposed to 

 include problematical forms. If we can not allocate an 

 animal on the basis of some supposedly fundamental 

 character, or if it falls on the basis of a single character 

 in a phylum from all the groups in which it differs in all 

 the others, we ignore that character entirely and take up 

 another. In every group each character has a definite 

 and restricted application, beyond the limits of which 

 it is quite valueless. The echinoderms are commonly said 

 to be pentaradiate, and the great majority certainly are ; 

 but certain genera, entirely or in part, possess three, four 

 (like most medusae), six, seven, eight or ten rays; we 

 recognize them as echinoderms just the same. Specimens 

 of the genus Limnocnida are commonly pentaradiate; 

 but we instantly recognize them not as echinoderms, but 

 as hydromedusae. The echinoderms we say have abund- 

 ant calcareous deposits in the skin, and often also in the 

 deeper parts of the body; the genus Pelagotluiria has no 

 trace of any calcareous deposits whatever, but no one 

 doubts that it is an echinoderm. 



These few obvious cases are selected from an almost 

 unlimited choice ; they show conclusively that any char- 

 acter, no matter how fundamental it may be, may sud- 

 denly become quite worthless, forcing us to depend en- 

 tirely upon other characters which in other cases are 



