PART II. 
STRUCTURE AND HABITS OF TRILOBITES. 
Internal Organs and Muscles. 
Granting that the trilobite is a simple, generalized, ancient crustacean, it appears justifi- 
able to attribute to it such internal organs as seem, from a stud)' of comparative anatomy, 
to be primitive. 
The alimentary canal would be expected to be straight and simple, curving downward 
to the mouth, and should be composed of three portions, stomodseum, mesenteron, and proc- 
todseum, the first and last with chitinous lining. In modern Crustacea, muscle-bands run from 
the gut to part of the adjacent body wall, so that scars of attachment of these muscles 
may be sought. At the anterior end of the stomodseum, they are usually especially strong. 
From the mesenteron there might be pouch-like or tubular outgrowths. 
The heart would probably be long and tubular, with a pair of ostia for each somite. 
In modern Crustacea, the chief organs of renal excretion are two pairs of glands in the 
head, one lying at the base of the antennae and one at the base of the maxillje. Only one 
pair is functional at a time, but these are supposed to be survivors of a series of segmen- 
tally arranged organs, so that there might be a pair to each somite of a trilobite. 
The nervous system might be expected to consist of a supraoesophageal "brain," com- 
prising at least two pairs of ganglionic centers, and a double ventral chain of ganglia with 
a ladder-like arrangement. 
Besides these organs, a variety of glands of special function might be predicted. 
Reproductive organs probably should occur in pairs, and more than one pair is to be 
expected. There is little to indicate the probable location of the genital openings, but they 
may have been located all along the body back of the cephalon. 
It may be profitable to summarize present knowledge of such traces of these organs 
as have been found in the fossils, if only to point out what should be sought. 
ALIMENTARY CANAL. 
Beyrich (1846, p. 30) first called attention to the alimentary canal of a trilobite, (Cryp- 
tolithus goldfussi,) and Barrande (1852, p. 229) confirmed his observations. A number 
of specimens of this species have been found which show a straight cylindrical tube or 
its filling, extending from the glabella back nearly to the posterior end of the pygidium. It 
lies directly under the median line of the axial lobe, and less than its own diameter beneath 
the dorsal test. At the anterior end it apparently enlarges to occupy the greater part of the 
space between the glabella and the hypostoma, but was said by the early observers to extend 
only a little over halfway to the front. Beyrich thought the position of the median tubercle 
indicated the location of the anterior end. 
Walcott (1881, p. 200) stated that in his experience in cutting sections of trilobites it 
was a very rare occurrence to find traces of the alimentary canal. The visceral cavity was 
usually filled with crystalline calcite and all vestiges of organs obliterated. There were, 
however, some slices which showed a dark spot under the axial lobe, which probably rep- 
resented the canal. In his restoration he showed it as of practically uniform diameter 
throughout, and extending but slightly in front of the mouth. 
