48 THE TRANSFORMATIONS OF THE ANIMAL WORLD 



the soft nature of these animals. Yet it is from 

 the various types of this branch that are derived, 

 according to Haeckel, all the groups of the higher 

 animals. 



The Molluscs would seem, at first sight, bound to 

 supply us with exact phylogenic documents by 

 reason of the abundance of their calcareous shells 

 in all geological strata. But, on the one hand, the 

 shell is a morphological organ of little importance, 

 and on the other, the four great orders of molluscs : 

 the Brachiopods, the Acephala, the Gastropods, the 

 Cephalopods, are already entirely differentiated in 

 all their characters from the very lowest strata of 

 the primary era. The evolution of the group is 

 therefore still earlier and the distant ages in which 

 it must have occurred have left us no traces of fossil 

 forms to guide us as to the origin and differentiation 

 of these beings. It may be said, however, that the 

 Brachiopods, which occupy the lowest rank among 

 these orders, swarm in primary times, while the 

 Lamellibranchs and Gastropods must have de- 

 veloped as two diverging branches of types very 

 near, at least, to the present Brachiopods. As to the 

 very much higher group of Cephalopods, comprising 

 Octopods, cuttle fishes, Sepias, and the modern 

 Calamaries, whose apogee goes back to secondary 

 times, they would be derived, according to Haeckel, 

 from the lower branches of the order of Gastropods ; 

 but, on the author's own showing, the transitional 

 forms are palseontologically totally lacking. 



The history of the Echinoderms is somewhat better 

 known, thanks to their abundance in most geolo- 

 gical strata and to their remarkable individual evo- 



