v.] METAMORPHOSES OF J t \ SECTS. 85 



tology has revealed that in the structure of its skull, 

 its teeth, its limbs, the horse is nothing more than a 

 modified Palaeotherium ; and though still with gaps 

 in certain places, many of the intermediate stages 

 of these modifications are already known to us, being 

 the Palaeotherium, Anchitherium, Merychippus, and 

 Hipparion," 



" All Echinoids," says A. Agassiz, 1 " pass, in their 

 early stages, through a condition which recalls to us 

 the first Echinoids which made their appearance in 

 geological ages." On embryological grounds, he 

 observes, we should " place true Echini lowest, then 

 the Clypeastroids, next the Echinolamps, and finally 

 the Spatangoids." Now among the Echinoids of the 

 Trias there are no Clypeastroids, Echinolamps, or 

 Spatangoids. The Clypeastroids make their appear- 

 ance in the Lias, the Echinolamps in the Jurassic, while 

 the Spatangoids commence in the Cretaceous period. 



Again 2 "in the Radiates, the Acalephs in their 

 first stages of growth, that is, in their Hydroid con- 

 dition, remind us of the adult forms among Polyps, 

 showing the structural rank of the Acalephs to be 

 the highest, since they pass beyond a stage which 

 is permanent with the Polyps ; while the Adult forms 

 of the Acalephs have in their turn a certain resem- 

 blance to the embryonic phases of the class next 

 above them, the Echinoderms ; within the limits 

 of the classes, the same correspondence exists as 

 between the different orders ; the embryonic forms of 

 the highest Polyps recall the adult forms of the lower 



1 "Embryology of Echinoderms," 1. c. p. 15. 



2 Mr. and Mrs. Agassiz : " Seaside Studies," p. 139.. 



