CHAPTER V 



Attitudinal Changes and Structural Modifications 



T X 7HEN in my book, Les Colonies animates et la formation 

 » » ^s organismes, I attempted to explain how the different 

 types of the Animal Kingdom had been evolved, it was not 

 difficult for me after having given a history of the branched and 

 segmented animals, to show, as other authors had pointed out 

 for each group in particular, that the Annelid Worms, in all 

 probability, were the progenitors of the Echinoderms, Molluscs, 

 Vertebrates, and, in consequence, of the degenerate derivatives 

 of the latter, the Tunicates. But although I had at that time 

 already pointed out the importance of tachygenesis, I had not 

 as yet realized the full consequences of this mode of hereditary 

 action, nor had I perceived one particularly powerful cause for 

 the modification of organisms, namely, the changes of posture 

 that have taken place in each species in the course of ages. 



To convince ourselves of the reality of these changes, we 

 need but cast our eye over the existing series of living forms. 

 Among the Crustaceans, A pus and other Branchiopods swim 

 with their ventral side uppermost and their dorsal side down- 

 wards ; the same is true of Notonectes among the Insects, and 

 its name indicates this position ; among the Cirripedes, Lepas 

 and its allies suspend themselves from floating objects by their 

 head, which is drawn out into a long peduncle ; while forms like 

 Badanus, closely related to them, obliterate, so to speak, this 

 same extremity against the rocks to which they closely adhere. 

 In the subdivisions of the Tunicates we observe the same 

 contrast between Boltenia and the other Ascidiacea, while the 

 Tunicates that have reverted to swimming retain the 

 normal position. Among the Echinoderms the common 

 sea-urchin has its mouth below and the anus above, 

 so that the five radial areas, bearing their organs of 

 locomotion, are erected vertically like the petals of a flower, 

 all five arising from the mouth and capable of reaching the 

 orifice opposite. But there are some species which dig cease- 



