VENTRAL STRUCTURE OF TRIARTHRUS 209 



they were doubtless originally post-oral in the Crustacea, as 

 is indicated from their innervation from the ventral nerve 

 ganglion chain and not from the archicerebrum of the prosto- 

 mium, or cephalic lobe. Besides, in the embryo of Limulus 

 all the appendages are post-oral, as shown by Packard.* 



The Mouth. Although the opening of the mouth itself 

 has not been observed in Triarthrus, there can be little doubt 

 as to its exact location, since it must have been situated in 

 the median line between the hypostoma and metastoma. As 

 both these organs are quite close together, the place of the 

 mouth would be as indicated on Plate VIII, figure 11, m. 

 Further corroborative evidence may be had from the genus 

 Calymmene, in which the mouth was determined by Walcott 

 (I. c.) to lie at the end of the hypostoma, opening obliquely 

 backward. 



The Metastoma. The lower lip, or metastoma, here repre- 

 sented for the first time, is generally clearly shown as a 

 convex arcuate plate just posterior to the extremity of the 

 hypostoma. On each side, at the angles, are two small ele- 

 vations, or lappets, which suggest similar structures in many 

 higher Crustacea, and apparently represent the entire metas- 

 toma in Apus and some other forms (Plate VIII, figures 9 

 and 11, met). 



The Anal Opening. In tracing the intestinal canal as pre- 

 served in Trinucleus, Barrande determined its posterior termi- 

 nation to be at the extremity of the pygidium, and Bernard f 

 has recently succeeded in reaching a similar conclusion, from 

 his studies of Calymmene^ in which the anal opening was 

 found just at the inner margin of the doublure of the pygid- 

 ium, in the median line. Plate VII, figure 1, of Triarthrus 

 shows the anus in the same position, outlined by a slightly 

 elevated wrinkled ring. 



* The Development of Limulus polyphemus. Mem. Boston Soc. Nat. Hist., 

 II, 1872. 



t The Systematic Position of the Trilobites. Quar. Jour. Geol. Soc. London, 

 L, 1894. 



14 



