402 THE AMERICAN NATURALIST [Vol. LIV 



is evidence from ontogeny and phylogeny of a backward 

 migration of the mouth, coincident with the same move- 

 ment of the eyes. In both cases this is probably due to 

 the enlargement of the anterior end of the mid-gut. 

 From the mouth the esophagus extends upward and for- 

 ward to the enlarged mesenteron which occupies the 

 greater part of the large cavity between the hypostoma 

 and glabella. The intestinal canal tapers backward, but 

 no differentiation of the posterior portion has yet been 

 made out. The anus is beneath the posterior end of the 

 axial lobe. The heart is elongate, chambered, branchio- 

 pod-like, and in the one species in which it is preserved, 

 extended from the middle of the cephalon to the anterior 

 end of the pygidium. The principal muscles were a dor- 

 sal pair of extensors, attached at the posterior margin 

 of the cephalon and anterior ring of the pygidium, and a 

 ventral pair of flexors, both with branches inserted in 

 each segment. All of these organs were within the axial 

 lobe. 



COMPAEISON WITH OTHER ArTHROPODA 



Having thus briefly stated the principal characteristics 

 of the trilobites, the method will be to indicate briefly 

 the similarities which exist between the trilobites and 

 other arthropods, and to show that there is nothing about 

 the bodily form or characteristics of the appendages to 

 negate the possibility of a derivation directly or indi- 

 rectly of all other classes from that under discussion. It 

 is obvious that in this short paper each class can be 

 treated but briefly. 



Crustacea 



The trilobites are themselves crustaceans, as is amply 

 proved by their biramous appendages. An attempt will 

 be made to show that they may have been ancestral to 

 the other crustaceans. 



In recent years it has been generally considered that 

 the Branchiopoda were more nearly allied to the trilo- 

 bites than any other living animals. Bernard, the chief 



