161 PHYLUM ECHINODEEMATA. 



It is therefore no more possible to explain the sinistral position 

 of the mouth in Echinoderms than it is to account for the same 

 phenomenon in Amphioxus. But that conclusion does not in 

 any degree diminish the importance of the character as an indi- 

 cation of affinity. On the contrary it increases it. For, if it 

 cannot be shown to be connected with habit of life or with other 

 peculiarities of structure in the animals presenting it, the pre- 

 sumption that it is a property which was possessed by the 

 ancestral matrix from which Echinoderms and Amphioxus have 

 emerged is increased. We are thus brought back to the ques- 

 tion which we touched upon on p. 116, are the Echinoderms 

 descended from asymmetrical or from bilaterally symmetrical 

 forms ? This discussion of the asymmetry of Amphioxus and 

 Echinoderms has elicited facts which are not without a bearing 

 upon this question. We have seen that in Amphioxus there is 

 hardly a single organ of the body which displays complete 

 bilateral symmetry at all stages of existence, and in the adult 

 traces of this asymmetry slight traces, it is true, but all the 

 more striking on that account are present (position of olfactory 

 pit and anus just to the left of the middle line, preoral hood and 

 other small distortions). We have also seen that the asymmetry 

 of one organ is entirely independent of the asymmetry of the 

 others. Very similar statements may be made about Echino- 

 derms : in these also development begins with a transitory 

 bilateral symmetry which is almost at once followed by asym- 

 metry, at least of the internal organs, and the asymmetry then 

 initiated is never completely got rid of, for the radial symmetry 

 of the adult is in all classes imperfect (least so in Holothurians), 

 and some of the adult distortions, such as the position of the 

 anus, recall the similarly slight distortions found in the adult 

 Amphioxus. Now the upshot of these considerations is to 

 make us pause in accepting as final the conclusion that the 

 ancestral Echinoderm was a bilaterally symmetrical animal. 



(4) The striking resemblance of the bipinnaria larva to the 

 tornaria of Enteropneusta has already been referred to (p. 99). 

 It is impossible to estimate its value, but it clearly cannot be 

 passed over in a discussion of this kind, and taken in conjunc- 

 tion with the other facts mentioned must be admitted to have 

 considerable weight. 



We have now passed in review all the points of resemblance 



