RADIAL STRUCTURE I ASYMMETRY. 163 



extraordinary features in the development of the gill clefts, of 

 the endostyle, of the head-cavities, the asymmetric positon of the 

 anus and olfactory pit, into relation with the asymmetry of the 

 mouth. The thing cannot be done. There is no sort of con- 

 nexion between these various asymmetries. They seem to 

 occur without rhyme or reason. The mouth which should be 

 a median structure is from the first on the left side ; the gill- 

 clefts, which are on the left in the adult, appear in the median 

 line and at once pass on to the right side ; the endostyle which 

 is a median structure in the adult appears as an entirely dextral 

 organ. 



In Echinoderms on the other hand the asymmetry of the 

 mouth is accompanied by changes in habit and by change of 

 other organs which seem to be connected with the change in the 

 mouth. The animal here becomes sessile or semi-sessile and 

 acquires an entirely different symmetry in which other organs of 

 the body participate in an intelligible manner. But though 

 we can understand to a certain extent that the shift of the 

 mouth might indent the left hydrocoel and bring about an 

 inequality in the posterior coeloms, no adequate attempt has ever 

 been made to show how the sessile habit and the radial structure 

 is connected with the shifting of the mouth on to the left side. 

 We have here three factors : the sinistral mouth, the radial 

 structure and the sessile habit. Can these factors be brought 

 into the relation of cause and effect ? 



(1) Can the sessile habit be regarded as the cause of the other 

 two, even if we accept the view that all Echinoderm classes 

 have passed through a fixed stage in their phylogeny. We can 

 only point out in reply that no such results have followed fixa- 

 tion in any other group of the Coelomata : they have not 

 followed in Cirripedes, Brachiopods or in Tunicates. 



(2) Can the left-handed mouth be regarded as the cause ? 

 In Amphioxus, the only other animal in which the mouth is 

 sinistral, it is accompanied neither by the sessile habit nor by 

 the radial symmetry. 



(3) Lastly, can the acquisition of radial symmetry, to whatever 

 cause due, have brought in its train the shifting of the mouth 

 and the sessile habit ? In the only other animals which can 

 lay claim to a radial symmetry, the Coelenterata, no such result 

 has followed. 





