llie Ventral Stracfiire of I'riarthni.^. — Beecher. 97 
second and third pairs of appendages, or the antennie and 
mandibles, with respect to the mouth, does not oft'er any seri- 
ous difficult}'. As shown by Lankester,* 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 prostomium, or cephalic 
lobe. Besides, in the embryo of Limulns all the appendages 
are post-oral, as shown by Packard. f 
The month. Although the opening of the mouth itself has 
not been observed in Triarthrns., there can be little doubt 
as to its exact location, since it must have been situated in 
the median line between the hj'postoma and metastoma. As 
both these organs are quite close together, the place of the 
mouth would be as indicated on plate v, figure 11 m. Further 
corroborative evidence ma}"- be had from the genus Calymene, 
in which the mouth was determined b}^ Walcott (1. c.) to lie 
at the end of the hypostoma, opening obliquel}- backward. 
The mefustoma. The lower lip, or metastoma, liere repre- 
sented for the first time, is generall}' clearly shown as a con- 
vex arcuate plate just posterior to the extremity of the hy- 
postoma. On each side, at the angles, are two small elevations, 
or lappets, which suggest similar structures in many higher 
Crustacea, and apparently represent the entire metastoma in 
ApHs and some other forms (plate v, figures 0, and 11 met). 
The anal o/ieiiiii(f. In tracing the intestinal canal as pre- 
served in Tri)ii(cletf.s\ Barrande determined its posterior ter- 
mination to be at the extremity of the pygidium, and Bernard* 
has recently succeeded in reaching a similar conclusion, from 
his studies of (fali/inene, in which the anal opening was found 
just at the inner margin of the doublure of the pygidium, in 
the median line. Plate iv, figure 1, of Triarthnts. shows the 
anus in the same position, outlined by a slight!}' elevated 
wrinkled ring. 
Ob.'ieroattori.s-. 
With these additional discoveries relating to Tn'arthrus, 
*Observations and Rcfiections on the Appendages and on the Nervous 
System of Apus cancriformis. Qiiar. Jour. Mic. Sci., vol. xxi, 1881. 
fTho Developmonl of Limulus polvphemus. Mom. IJoston Soc. Nat. 
Hist., vol. n, 187-2. 
^TIk' Svstomatic Position of llu' Triiobitcs. Quar. .lour. (icol. .Soc, 
voi. I,. 18!M. 
