A. H. Clark — Pentamerous Symmetry of Crinoidea. 355 



disappear, since it is quite useless for the collection of food — 

 we are forced to look for some phylogenetic cause. 



Young crinoids are, as has been mentioned, bilaterally sym- 

 metrical ; all the four-rayed adults of the comatulids so far 

 observed are bilaterally symmetrical ; the normal five-rayed 

 adults are the equivalent of the four-rayed adults, plus the 

 interpolation of an additional ray between the two of the 

 anterior pair. The more or less frequent reversion of the 

 five-rayed crinoids to a four-rayed form (a permanent condition 

 in Tetracrinus) would appear to be an index pointing back 

 along the phylogenetic path travelled by the race: and this 

 index becomes all the more significant when we glance at the 

 record of six-rayed variants in which there "is no such uni- 

 formity of place of interpolation. 



It would appear as though the evidence pointed to an 

 original bilateral condition among the adult crinoids in which 

 there were two pairs of body processes that had become, 

 through similarity of function, the same ; in other words, a 

 bilateral condition which had become as well, through induced 

 radial symmetry due to inactive habit, quadriradial. We may 

 readily imagine that, by sporadic variation, an additional ray 

 might sometimes be produced between the two of the anterior 

 pair, especially in animals of such remarkable regenerative 

 powers as the crinoids. This additional ray would be just 

 like those on either side of it ; and in a sessile animal it would 

 cause no inconvenience but would, on the contrary, be of 

 distinct advantage in furnishing a very marked increase in 

 food-collecting power. Animals with this additional ray 

 would therefore have, in the case of sessile forms like the 

 crinoids — passive feeders which must wait for their food to 

 come to them — a great advantage in the struggle for existence, 

 and we might well imagine that nature would be quick to 

 take advantage of such a variant, and to fix permanently this 

 pentamerous condition. 



If we can admit that the four-rayed condition which, so far 

 as is known, always arises from the loss of the anterior ray, is a 

 true phylogenetic index — and there is no good reason why we 

 should not — then we would be led to the conclusion that the 

 crinoids are primarily animals possessing a bilateral symmetry, 

 with two pairs of similar appendages, an anterior and a poste- 

 rior, upon which a secondary pentamerous symmetry has been 

 superposed, due to interpolation of one half of one of the 

 bilaterally symmetrical pairs of appendages between the two 

 components of the original anterior pair. 



It might be urged that supernumerary paired appendages 

 would be unlikely to put in an appearance except in pairs ; but 

 we find instances of just that thing in other widely different 



