700 



ARTHROPODA 



PHYLUM VII 



The alimentary canal, discovered by Beyrich and Volborth, begins at 

 the mouth and then curves over backward beneath the glabella, and extends 

 parallel with the dorsal test to its termination in the anal opening at the 

 posterior end of the pygidiura (Fig. 1341). 



Most of the recent advances in the knowledge of Trilobite structure have 

 come from the study of numerous very perfectly preserved specimens of 



Fig. 1342. 



Trmrthrus hecki Green. Utiea Slialu (Oi(lo\'ician) ; Rome, New York. A, Dorsal, and B, Ventral aspect, 



2/i (after Ueeuher). 



IViarthnis hecki Green, from the Utica Shale (Ordovician), near Eome, New 

 York. Undoubted antennae in this form were discovered by Valiant, and first 

 announced by Matthew in 1893. Subsequently a series of papers was 

 published by Beecher on the detailed structure of this Trilobite, Avhich is now 

 the best known of any species, and necessarily forms the basis of much of the 

 following summary of ventral organs. 



In the median line anteriorly, there is first the hypostoma or upper lip, at 

 the end of which, and opening obliquely backward, is the mouth (Walcott, in 

 Calyrmw). In Triarihrus the lower lip, or metastoma, is a convex arcuate 

 plate, just posterior to the extremity of the hypostoma. At the angles on 

 either side are two small elevations, or lappets. 



